Continuity: Willow is turned into a ghost. Throughout the entire episode, she can't touch anything (can't turn the pages of a book, etc.), yet towards the end when she and Giles go to Ethan's shop, and she leaves, you can hear that she opened and closed the door, even though she can walk through walls and can't actually touch the door. The curtain moves when she leaves too.

Continuity: The vampire Angel fights in Buffy's house was never invited in.

Other: When Buffy and Willow find the picture of the noblewoman in the Watcher's Diaries, Buffy asks "Who is she?" and Willow replies "It doesn't say", but if you watch closely you can see that during the brief shot of the book her name is visible next to her picture.

Character mistake: While researching Angel's past, Willow comments that he was still human in 1775. But he was already a vampire by then, having been sired in 1753.

Plot hole: Giles tells Willow to leave him while he extracts information from Ethan on how to break the costume spell. By the time Giles smashes the statue Willow has already found the others, who had left the house where she thought they were and are hiding in a completely different area of the town. No matter how long it took Giles to beat the information out of Ethan, there is no way Willow could have gotten back to the house, realised they were gone and tracked them down before the spell was broken. It is also impossible she could have happened across them - they were hiding, after all.

Audio problem: Just before Willow got turned into a ghost, she started to talk, but it wasn't her voice that was talking, it sounded like Cordelia.