Girl Trouble / School Dance - S3-E18
Continuity mistake: Besides the fact that when Helga is sharpening her pencil, the desk arrangement changes, one of the students visible when she looks at Arnold is... Helga. This happens because the shot is taken from the episode "Helga's Love Potion."
Visible crew/equipment: A crew member can be seen to the left of the frame when Eric walks into Simon and Ruthie's room after listening to their conversation about not being punished for leaving the house, and getting lost while trailing the mailman. (00:36:15)
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.
Kingdom Come - S1-E6
Other mistake: At the start of chapter 12 in the episode "Kingdom Come," Frank returns home via taxi. In front of his house in Seattle, WA, there is a blonde woman walking a Rottweiler dog. This same blonde and Rottweiler can be seen walking by the crime scene in San Francisco in the "Gehenna" episode.
Continuity mistake: In Episode 5, "Live Hard, Die Young and Leave A Good Looking Corpse": As Lillie is preparing to cut Zane's throat with her fingernail, she stands with her hand raised above her head for a while. When seen from the front, her hand is balled into a fist with the index finger extended, but when seen from the back all her fingers are extended. The view alternates like this several times, and each time the hand varies between fist and outstretched fingers.
Plot hole: When Nicky goes into prison in Mexico to give a criminal a shot of insulin, two errors are seen. 1) The needle and syringe are far too big for an insulin shot, which takes a one ml syringe and a small sub-cutaneous needle. 2) When we first see the syringe, there is a small amount of insulin in it, but it grows the more times we see it, until a large syringe is two-thirds full - insulin is enough in a one ml syringe.
Episode #2.5 - S2-E5
Visible crew/equipment: In the house when Dinah is asking Rose the location of the other children you can see a piece of equipment (probably the boom mic) reflected in the top of the window between them. (00:19:37)
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)