Close Encounters of the Beans Kind - S3-E8
Revealing mistake: When Louis and Beans fall from the sky on the bike, if you look carefully, you will see that they're both dummies.
Influenza: The Musical - S2-E21
Revealing mistake: During the song "Sixth Period", while Coach Tugnut is "running" on the treadmill, if you watch the treadmill, you'll notice that it isn't actually moving at all.
Close Encounters of the Beans Kind - S3-E8
Revealing mistake: When Mason is chasing the bike, the bike is going slow enough so that Mason should be able to catch it.