When Buffy first runs over to Giles at her party, he is holding a plate with cake on it. Moments later it is gone, and Buffy uses his lack of cake as an excuse to make Riley leave when things are awkward.
As Giles drinks with Ethan Rayne in the pub, the beer in his glass changes levels. It's full when Giles faces the camera and it's dramatically emptier when the camera points across the table toward Ethan.
When Giles is reading the book about when a demon will be showing up, the pages of the book are obviously blank.