The Banker and the Bum - S1-E20
Revealing mistake: Jonathon is supposed to be invisible when the bum is talking to the limo driver but he walks past the driver and casts a blatant shadow on him.
One Fresh Batch of Lemonade: Part 1 - S1-E6
Revealing mistake: Tire marks and other items from previous takes are visible on the pavement when the motorcycle crashes. You can see black skid marks where the vehicles crash on cue.
Another Song for Christmas - S1-E13
Revealing mistake: The smoke from the supposedly broken vehicle is obviously not coming from the tailpipe. It is coming from the trunk.