Out Of Mind, Out Of Sight - S1-E11
Continuity mistake: When Buffy finds the invisible girl's "home" in the ceiling and picks up her flute, the position of everything on her bed changes between the initial closeup we saw and the shot of Buffy picking up the flute. Ex. In the initial closeup, the flute is on an open case, while in the shot where Buffy picks it up the case is closed. All the sheet music is in a slightly different position. A cylindrical object appears next to the flute that wasn't there in the initial closeup, etc.