Season 3, Episode 1, "Anne", at 39 minutes and 42 seconds.

When: Buffy is in the universe that has time moving faster, and all the kids working for this group of demons. She is fighting on a big box, or stage.

The Slipup: An action is filmed from two angles, and you can clearly see two guys sitting in the background, one with tennis shoes on and some normal clothes, and the other behind a camera. It is right when Buffy throws a demon off the stage/box she is on. Paused at 39:42 you can see them easily.