Revealing mistake: During the fight scene at the end when Buffy is fighting the demons you see her throw one of the demons off the platform. If you watch him fall you can see him land safely on a conveniently placed mat. (00:38:10)
Revelations - S5-E22
Revealing mistake: Just before Teal'c and O'Neill get to the engine room door, Carter can be seen hiding around the corner. (00:27:40)
Revealing mistake: The footage of Monkey Bulk doing flips is just one backflip forwarded and reversed several times - as evident by the movements of the people behind them.
Magical Express Murder Case: File 1 - S2-E10
Revealing mistake: The train has been evacuated and the manager is looking for the troupe leader. There is something wrong in the train behind him though, because the main body of the train is floating over the wheelset. (00:14:00)