The Gang Gets Held Hostage - S3-E4
Revealing mistake: When Liam is stabbed in the arm, as he turns around to walk away, you can see the outline of his real arm wrapped behind his back, under his robe for about one full second, revealing that the arm he was stabbed in was a fake prop-arm.
The Gang Gets Invincible - S3-E2
Revealing mistake: When Dennis is running in slow-mo to catch a pass, the QB throwing him the ball is visibly in the background the whole time and never actually throws the pass. (00:16:00)