ABBOTT AND COSTELLO. BtVS Intertext/Allusion.

In Fear Itself (Season 4) on the night before Halloween, during a pumpkin-carving get-together at Xander's place, Buffy leaves the gang a little early to spend some time alone. Outside, Buffy knocks a demonic-looking figure to the ground. Still recovering from Parker's rejection, she barely reacts to the realization that it was merely a guy in his Halloween costume.

The following dialogue is taken from the Buffy Dialogue Database

Xander:  "The joint’s not jumping.  Where is everybody?"
Mechanical laughter comes from a head with one eye hanging from its socket sitting in a punch bowl on a table next to the door.
Oz:  "Follow the signs."
Buffy looks at the severed head:  "Terrifying.  If I were Abbott and Costello this would be fairly traumatic."

Bud Abbott and Lou Costello were the most popular comedy team of the 1930's and 40's, famous for the screamingly funny "Who's On First?" routine.

We're not Abbott and Costello experts, but this would seem to be a reference to the numerous monster movies (such as Abbott and Costello Meet Frankenstein made by the comedy duo (Bud Abbott and Lou Costello) in the late 1940s and the 50s.

Xander also referenced the duo in "Go Fish."

In Go Fish (Season 2) Willow is being leaned on by Principal Snyder to give a swim team member a better grade than he deserves, since they are soon to win the State Championships. To complicate matters, something is eating the swim team one by one, leaving only their skins. Xander decide to go under-not-a-lot-of-cover in the swim team, where he finds out they are being fed steroids by the coach. The steroids are turning the team into fish monsters, which eat the school nurse. The coach tries to give the fish-boys Buffy, but she throws him to them instead, and they escape into the ocean. — Short synopsis by Bruce.

The following dialogue is taken from the Buffy Dialogue Database.

Willow: I think it's festive. A party with nature.

Cordelia: Well, it's the team's choice. It was their victory.

Xander: Team? Swim team. (chuckles) Hardly what I'd call a team. The

Yankees. Abbott and Costello. The 'A'. Now, those were teams.

"The Yankees... Abbott & Costello... the 'A.'"  The Yankees are, of course, New York's American League baseball franchise.

--Melissa Lamb