The Agent Of Year Zero - S4-E14
Revealing mistake: During the scene when Mr. Lyle is talking to the General (butcher character), his glove that he wears on his hand is removed to make a point about his thumb having been cut off. In the next shot, we see Mr. Lyle walking around the desk and putting his glove back on, and you can see his thumb folded under his hand as if the actor was trying to hide it and did not realize it could been seen from the camera angle.
Revealing mistake: Throughout the entire series there is one staircase upstairs and two downstairs. And, they more than likely are not the same staircase because the two downstairs are on opposite ends of the house.
The Big Scoop / Harold's Kitty - S2-E3
Revealing mistake: Harold's Kitty: When Arnold kicks the ball during the kickball game, he starts running, and he is layered over Helga, even though he is supposed to be behind her. (00:12:20)
Take Me out to the Ballgame - S4-E3
Revealing mistake: When the fat guys are running down to stop the bad guys, you can see the blue safety mat that they land on just after jumping. (00:37:00)
Episode #1.2 - S1-E2
Revealing mistake: Towards the end of the episode when Dinah, Lloyd and Harvey are walking to school you can see a man and a woman stood at the end of their driveway watching filming. (00:19:28)