During the earthquake, Spike is next to a mirror and you can see his reflection.
When Giles is singing "Behind Blue Eyes" at the coffee shop, his vocals are clearly augmented by a synthetic/echo effect, despite the fact he isn't using any type of microphone. This is obviously due to the performance being pre-recorded.
In Season 4, Episode 18, at around the 33:13 mark, Willow is seen reading aloud from a book. The way her mouth is moving clearly doesn't align with the words she is saying.
When Xander and Anya are going to Riley's room to rescue Riley and Buffy, there are a pair of hands "helping" with the vine attack on Anya.
When the spell to summon the poltergeists is interrupted and the table thrown across the room, the top-down wide angle shot that follows shows a member of the crew in the corner by the bedroom sink.
As Anya stands up after being thrown onto the couch from the balcony, the camera operator is visible, reflected in the glass-front bookcase behind her.