The trio of nerds invented an invisibility ray, using the Illuminata diamond, stolen from Sunnydale museum in Smashed. They accidentally shot Buffy with it in Gone, and she became invisible. Anya and Xander discovered that the invisibility ray was actually unstable, and everything it was used on eventually disintegrated.

Dawn makes a comment to Buffy in Entropy about her huge breakfast, “Are we going international?” This is a reference to a chain of restaurants called the International House of Pancakes. They sell massive breakfasts. IHOP are not as international as they’d like to be: they only have branches in …

When Willow goes to Oz’s cage to wake him with breakfast, she finds Veruca in the cage with him. If you look closely while they’re still asleep, you can see the flesh-coloured underwear on both Oz and Veruca. When Veruca awakes, she is wearing pants and a bra, but when …

The Scoobies pull up to the church where the vampires are and leap out of their van. A policeman stops them from going inside, and Willow starts to protest. We then cut to a slightly different angle and you can see that Willow says an entire sentence without moving her …

The Pit was an area in the Initiative’ huge headquarters where experiments were performed on vampires and demons. The walls were covered with tin foil for some reason.

An earthquake in 1812 caused the Sunnydale Mission to sink below ground. Xander accidentally unearthed it, causing the spirit of Chumash Native Indian Hus to be released upon the community, and Xander to get syphilis in Pangs.

Sunnydale Arms was an abandoned boarding house in Prescott Lane in Sunnydale. The Watchers’ Council bricked up the windows to use it as the site for Buffy’s cruciamentum on her eighteenth birthday in Helpless.

St. Vigeous led a crusade of vampires. The crusade “swept through Edessa, Harran, and points east”, according to Giles in School Hard. The Night of St. Vigeous is a time when vampires’ powers are at their greatest, though no reason is given.

Spontaneous combustion is when someone or something bursts into flames for no apparent reason. In Once More, With Feeling, many people danced crazily before combusting - a kind of suicide.

In Enemies, Angel and Buffy went on a date to the cinema where they saw Le Banquet d’Amelia in Enemies. This is not a real film, and was not what they were expecting:
Buffy: “That was very … artistic.”
Angel: “Yeah.”
Buffy: “Wasn’t what I expected. I’ve never actually seen… Well, …

In Revelations, Gwendolyn Post said Giles did not have the book Labyrinth Maps of Malta in his library.

In A New Man, Giles met Professor Walsh for the first time. He said, “You’re hard to find. These halls are quite the labyrinth. I felt like Theseus and the Minotaur in the . . . labyrinth.” Giles is referencing Greek mythology, in which the Minotaur (a creature with the …

Killer of the Dead is a poison used to kill vampires. It causes extreme pain, and a long death. Faith shot Angel with an arrow dipped into the poison in Graduation Day (Part 1). The only cure is for a vampire to drink the blood of a vampire Slayer. After …

Translated from Latin, tabula rasa means ‘Clean slate’. It is a forgetting spell which involves burning a lethe’s bramble, and dipping a crystal into the flame. When the fire burns out, the crystal turns black, and memories are erased. Willow performed the spell in the episode Tabula Rasa, but it …

Tachicardia is an overactive heart, resulting in a racing pulse. Riley suffered from it as a withdrawal symptom from the drugs that The Initiative had been feeding him. It was fixed in Out of My Mind.