Kelly Donovan

Nicholas Brendon has an identical twin brother called Kelly, who is often mistaken for Nick by fans. Kelly has done stunt work on Alias and played Jacob in MTV’s Undressed. Kelly doubled for Nick in the episode The Replacement, when Xander was split in two. Nicholas mostly played both characters but Kelly was used as a double, and had a bit of dialogue when the two were in the same frame.
In some scenes in Intervention, Xander was played by Kelly Donovan because Nicholas was sick.
Here’s what Nick said on the message boards at his official site:

Stay tuned

Here’s a stage direction from the script for The Replacement:

Two Xanders

Jane Espenson, the writer of the episode The Replacement, discussed the episode in an interview with the BBC. She said:

Michael Bailey Smith

Michael Bailey Smith, who played the lava-headed Toth in The Replacement, has also appeared in Men in Black II, My Favorite Martian, The Fantastic Four, and A Nightmare On Elm Street: The Dream Child. He appeared in an episode of She Spies called ‘The Replacement‘ and played Belthazor in Charmed. Michael played G’Dok in the Babylon 5 episode ‘A Day in the Strife’.

Toth

Toth was a strong, glowing, robed demon who was the last surviving member of the Tothric Clan. He used tools or devices (such as the Ferula Gemina) instead of fighting barehanded Toth attempted to split Buffy into her two separate parts (the Slayer and Buffy) in The Replacement, for no apparent reason. He failed and was eventually killed by the Slayer when he attacked her at Xander’s apartment.

Anya’s apartment

We see Anya’s very old-fashioned apartment for the first time in The Replacement, and see it again in Into the Woods.

Anya’s gun

We discover in The Replacement that Anya owns a gun, though we never see it again after that episode. Guns are usually frowned upon in the Buffyverse (seen in Phases, Homecoming, Flooded, As You Were and Touched), but it seems likely that Anya would have one.

Country boy

In The Replacement, Xander wears an ‘Iowa’ t-shirt. Riley is from Iowa.

Doubles

There have been a number of episodes with two of the same character in one shot. Willow met her vampire self in Doppelgängland, Xander had a double in The Replacement, we see two Spikes in Sleeper and the First takes on Buffy’s form in numerous episodes, including Chosen.

Evil twin

In The Replacement, Xander tells Willow to see how she handles it when she gets an evil twin. She says, “I handled it just fine”, a reference to when she met her vampire self in Doppelgängland.

Ice Capades

In Helpless, we discovered that Buffy’s father Hank took Buffy to the ice capades every year on her birthday (”Look, I know you guys think it’s just a big, dumb, girlie thing, but it’s not. I mean, a lot of those skaters are Olympic medal winners. And every year my dad buys me cotton candy and one of those souvenir programs that has all the pictures, and okay, it’s a big, dumb, girlie thing, but I love it”). Buffy is upset as her father can’t make the date in Helpless because of work commitments, though she didn’t go with him the year before (Surprise) either. After this, Buffy is busy every year on her birthday, either hunting down the Giles-demon (A New Man), protecting her sister Dawn (Blood Ties) or being trapped in a house by Halfrek (Older and Far Away).
In The Initiative, Willow tells Riley that Buffy “likes the ice capades without the irony”. Willow admitted she was sick on Woodstock at Snoopy on Ice when she was little in Helpless.
Buffy’s love of ice skating is also seen in the episodes Angel, What’s My Line, Part One and The Replacement (when Riley says he loves her bad ice skating movies obsession).

Intrusion

When Willow bursts through Giles’ front door in The Replacement, Giles says, “I swear this time I know I had that locked.” We saw Spike come through the door in a similar manner in the episodes The Harsh Light of Day and The Yoko Factor.

Mannequin

Spike gets a mannequin from the Sunnydale rubbish dump in The Replacement and later pretends it’s Buffy, a sure sign of his obsession with her. In Triangle he practises giving a speech and some chocolates to Buffy using the mannequin, to apologise for taking Buffy to see Riley at the vampire den.

Snoopy dance

Xander ‘convinces’ Willow in The Replacement that he is genuinely Xander by performing his Christmas Snoopy dance - first mentioned in Passion. Nicholas Brendon has performed this dance on numerous occassions for fans at conventions.

Teenage daughter headache

The first episode to suggest that there was a problem with Joyce’s health was The Replacement when she told an arguing Buffy and Dawn that, “this must be my teenage daughter headache”. Joyce collapsed in Out of My Mind and was taken to the hospital where she was declared fit. They doctors were wrong - they discovered she had a brain tumour in Shadow and she eventually died of an aneurysm in I Was Made to Love You.

Ferula Gemina

The Ferula Gemina looks like a stick, but has the power to split a person in half, putting positive and negative personality traits into two separate bodies. Neither body can survive without the other. Toth intended to kill the ‘Buffy’ part of Buffy in The Replacement, so the Slayer would be killed but he hit Xander instead.

Let the spell be ended

Jonathan’s demon-making spell in Life Serial is ended with the words “let the spell be ended”. In The Replacement Willow enjoined the two separated Xanders using the same words.

Ooftah

Ooftah is the goddess of childbirth. Giles used a statue of Ooftah to hit Toth in The Replacement. Willow agreed the statue was a wise choice as, “She’s got some nice heft to her.”

Babylon 5

Babylon 5 has been referenced a couple of times in Buffy. In The Replacement, Xander mentions his Babylon 5 commemorative plates. In Entropy, we learn that Jonathan used to collect Babylon 5 novels. Danny Strong, who played Jonathan, attended a Buffy Convention in London in 2003, alongside Babylon 5 star Julie Caitlin Brown.
Robin Sachs (Ethan Rayne) played Hedronn in the show and Robin Atkin Downes (Machida) played Byron, a poetic telepath, in Babylon 5.

Charlie Brown

Willow and Xander have a Christmas tradition of watching A Charlie Brown Christmas together. This was first mentioned in Passion. The cartoon features the Snoopy Dance, which is something we see Xander do in The Replacement.
In All the Way, Spike says, “Great Pumpkin’s on in twenty.” He’s referring to Charles Schultz’s animated Peanuts Halloween special It’s the Great Pumpkin, Charlie Brown.

David Sedaris

In The Replacement, Xander follows his double to his workplace. Whilst hiding, Xander is hit by the door of a Porta-cabin opening. The worker who comes out of the cabin has a name tag on his hat - Sedaris. This is probably a reference to humorist writer and columnist David Sedaris. Several compilations of Sedaris’s columns have been published in books such as Naked, Barrel Fever, and Me Talk Pretty One Day.

Star Trek

There are many cast/crew links between Buffy the Vampire Slayer and the long-running cult sci-fi show Star Trek. Armin Shimerman (Principal Snyder) played Quark in Star Trek: Deep Space Nine for seven years. Dominic Keating, who played Blair in the episode Helpless, later went on to star in Star Trek: Enterprise as Lt. Reed. Jennifer Hetrick, who played the teacher Ms. Moran in Homecoming (whom Buffy asked for a reference) played the girlfriend of Jean Luc Picard in Star Trek: The Next Generation. Buffy writer Jane Espenson wrote an episode of Star Trek: Deep Space Nine called ‘Accession’. Star Trek has also been referenced numerous times in Buffy the Vampire Slayer:

• In Prophecy Girl, Xander says, “Calm may work for Locutus of Borg here, but I’m freaked and I intend to stay that way.” Upset by Giles’ reserve, he is referencing Star Trek’s emotionless cyborgs from the episode ‘The Best of Both Worlds’. Locutus was the name given to Captain Pickard (Patrick Stewart) when he was captured, and ‘assimilated, by the Borg.
• In Homecoming, Cordelia woos the nerds at Sunnydale High by saying, “Are you kidding? I’ve been doing the Vulcan death grip since I was 4.”
• In Consequences, Cordy calls Wesley, “Giles the next generation” in a reference to Star Trek: The Next Generation.
• In Out of My Mind, Buffy says, “You’re like my fairy godmother and Santa Claus and Q all wrapped up into one… Q from Bond not Star Trek“.
• In The Replacement, the two Xanders say, “Kill us both Spock” - a reference to a Star Trek episode where Kirk is split two - one being good and one bad.
• In Flooded, the nerds vote with the Star Trek Vulcan salute, which is the same salute that Cordelia used to impress the ‘geeks’ in Homecoming.
• In Smashed, Spike tells the nerds, “You can play holodeck another time” - he means the virtual reality technology used in Star Trek.
• The nerds compare Buffy’s time loop in Life Serial with an episode of Star Trek: The Next Generation called ‘Cause and Effect’ (Andrew: “I just hope she solves it faster than Data did on the ep of TNG where the Enterprise kept blowing up.”)
• In As You Were, Buffy says. “they’re like really mean Tribbles”, referring to the popular, but quick breeding, pets on board the Starship Enterprise.
• After her visit to the nerds’ ‘lair’ in Doublemeat Palace, Willow says that they had numerous pictures of the “Vulcan women from Enterprise“. She’s referring to Jolene Blaylock, who played T’pol in UPN’s Star Trek show.
• The episode Normal Again is similar to the season five episode of Star Trek: Deep Space Nine called ‘Far Beyond the Stars’. In that episode, Captain Benjamin Sisko imagines that he is a science fiction writer living on 1950s Earth and writing about a station full of aliens called Deep Space Nine. He hallucinates that the people he knows in the 1950s are futuristic aliens and is thrown into an asylum.
• In Seeing Red, Andrew references Star Trek: The Next Generation when he discusses who’s boss of the nerds: “Warren’s the boss. He’s Picard, you’re Deanna Troi. Get used to the feeling, Betazoid.” In that episode, Xander realises that the nerds had love poems in their lair written in Klingon.
• In Grave, after the Magic Box has been destroyed, a William Shatner book can be seen on the floor.
• In Conversations with Dead People, we learn that Andrew learned Klingon (a language in Star Trek) from a dictionary in two and a half weeks.
• In Dirty Girls, Andrew hilariously confuses Faith’s murder of a Volcanologist with a Vulcan:
  

  Andrew: “Nobody was immune to her trail of destruction. Not friends, not family, not even the most pacifist and logical of races…”
  Amanda: “What the hell are you talking about? I thought Faith killed a volcanologist.”
  Andrew: “Silly, silly Amanda. Why would Faith kill a person who studies Vulcans?”

WP (Widespread Panic)

A black and white oval sticker with the letters “WP” can often be seen in Sunnydale. The sticker is for a band called Widespread Panic. Examples of episodes in which this is seen include:
Inca Mummy Girl: behind Xander when he says to Ampata, “Why’d you run away?”.
Halloween: next to Cordelia when she’s dressed as a cat and talking to Oz (on Oz’s locker door), and when Willow walks into the library when she’s a ghost, there is one on the bulletin board to her right. There is also a sticker on the bathroom wall, seen behind Buffy’s right shoulder when she an Willow look through the book.
Reptile Boy: at the beginning of the episode, behind Xander’s left shoulder in Buffy’s room.
Bad Eggs: behind Jonathon’s right shoulder on the locker when he is being attacked.
Surprise: on the locker behind Cordelia when she is talking to Xander and behind Xander when he calls Giles a party weasel. A colourful Widespread Panic poster can also be seen behind Joyce in Buffy’s dream.
Phases: when Buffy is in the Bronze looking for the werewolf, a WP sticker can be seen on a pillar behind her right shoulder.
Bewitched, Bothered and Bewildered: poster in Xander’s room and behind Xander when Harmony tells him he should learn a second language so more girls can reject him. A sticker can also be seen on the locker behind Cordelia when Harmony is telling her off for breaking up with Xander.
Becoming (Part 2): at the beginning of the episode, when the cop is arresting Buffy in the hall, you can see a WP sticker on the lockers behind her.
Dead Man’s Party: on a guitar case behind the drums during the party.
A New Man: behind Giles when he’s at Buffy’s party.
Wild at Heart: a colourful WP poster (not the sticker) can be seen to the right of Oz’s bedroom door.
The Yoko Factor: in Buffy and Willow’s dorm room.
The Replacement: on the payphone that Xander uses to call Buffy.

Seen at 15.17 minutes:

In the scene with Spike and the mannequin, Spike has a tan line on his arms, where his t-shirt has been. Vampires don’t tan - they dust.

Seen at 25.54 minutes:

When Xander leaves Willow’s room to go to Anya’s he doesn’t have his multi coloured shirt on, as he left it at Willow’s place. When we see him arrive at Anya’s, he has the shirt on again.

Seen at 26.13 minutes:

It’s strange how the answering machine only plays back the middle part of Xander’s message - and the exact part that he needed to hear!

Seen at 28.04 minutes:

Anya says that she has never been hurt really bad before since she became human, but she was stabbed badly through the hand in the episode Where the Wild Things Are.

Seen at 40.26 minutes:

Xander mentions his Babylon 5 commemorative plates. They don’t exist - they were planned, but never released.