Phoebe

Witch - S1-E3

Trivia: On the cheerleading banner, the year is proclaimed to be 1996, which was when it was filmed. But the episode actually aired in March 1997.

Phoebe

The Initiative - S4-E7

Plot hole: Spike must have had his chip implanted while he was unconcious. If so, how did he attack The Initiative scientists without any pain when he tried to escape? You could argue that they programmed him to be capable of violence within The Initiative only, but why would super smart scientists program an evil being to be able to escape at the first opportunity? Joss Whedon has acknowledged this as an oversight.

Phoebe

The Initiative - S4-E7

Revealing mistake: Several times during Spike's escape from The Initiative, his reflection can be seen in the glass. Vampires aren't supposed to have reflections. (00:13:00)

Phoebe

Same Time, Same Place - S7-E3

Other mistake: We're shown a variety of photographs and pictures of different members of the gang at one point. One of those, the one of Xander and Anya, is actually a shot from the episode "Something Blue". I doubt very much that during the events in that episode, someone conveniently decided to take a photograph.

Phoebe