Other mistake: In the episode 'Anne' Kenn tells Buffy that time in his hell dimension passes more quickly, that one day on Earth is the same as 100 years in his dimension. This means time goes roughly 36,000 times quicker. When Lily falls into the portal, it's at least 10 seconds until Buffy jumps in after her, but Lily is still just sitting there looking stunned. If times moves that much faster, 10 seconds would become over 4 days. Would Lily still have been just sitting there?
Continuity mistake: Near the start of 'The Wish', the scene where Cordelia is talking to the Cordettes and meets Anya. During the close-ups of Harmony, Anya is standing to camera right of the brunette Cordette behind her. A few seconds later when the shot is further away Anya is standing camera left of the Cordette.
Plot hole: In 'Graduation Day pt 2' the Mayor has his vampires stand at the bottom of the steps leading out of the school to stop any escaping students. The thing is, all the parents managed to escape out the side exits. Since the Mayor was expecting all the students to try and run too, why didn't he have these exits covered?
Continuity mistake: When the female watcher is being destroyed after Buffy cuts off her arm. Her sleeve is empty, which is not possible, as Buffy could not have removed the arm but not the sleeve. Also the arm on the floor is removed at the elbow, but the watcher has lost the entire arm in a previous shot.
Continuity mistake: When Buffy is walking forwards through Angel's dust in 'The Wish' there's a vampire running towards her, only a few steps behind. Then there's a shot of her walking forwards taken from another angle, then when we can see behind her again the same vampire hasn't moved forwards at all, he's still running from 3 steps behind her.
Factual error: There is a short picture of a 17th century German newspaper. The actual text however not only is written in modern High German as opposed to Mid-High German, but also is full of errors (mixing female articles with male nouns is only one example). Funnily enough, Giles' incantation at the end of the episode is in correct German.