Revealing mistake: During the training montage, right as Mac is about to smash the bottle over Charlie's head, you can see Charlie stop what he's doing about a half-second before being hit, and brace himself for the impact, despite not being able to see it coming. This is confirmed on the DVD commentary.
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.
Revealing mistake: As Mac says "I'm very worried about these guys" and exits the bar, there's a shot right through the door and it's not the bar inside at all, just a wall with some pictures on it, much closer to the door than it should be.
Revealing mistake: Right before Mac and Dennis start to fight in the mall, people in the store stare at the camera and a woman poses at the camera.
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)
Revealing mistake: As Mac and Dennis are wheeling themselves through the mall, they pass by a clothing store where a man and woman are standing by the entrance. The woman notices the camera and waves at it. (00:15:22)
