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.
Full Moon / Student Teacher - S4-E1
Revealing mistake: Student Teacher: When Olga says, "But, nothing, baby sister!" before giving Helga detention, Rhonda and Brainy disappear for one frame. (00:17:17)
Revealing mistake: Throughout Series 2 Eve's reptilian hand is a glove made out of rubber and you can tell whenever she bends her wrist and the "skin" twists and wrinkles. An obvious example of this is in Episode 6 when she has finished going through "Accelerated Learning" and removes the listening device, bending her wrist and the glove in the process.
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)