The Treaty - S4-E7
Revealing mistake: Tom is talking on the phone in Ron's office. When he turns the phone at an angle the call home screen that has the end button, the speaker button, etc. turns to its side, which means it was a picture - he wasn't actually calling anyone. (00:02:25)
Revealing mistake: Season 2, episode 4 (Duets): When Brittany lifts Artie from his chair and carries him to her bed, right before she puts him down, he is obviously lifting his feet to prevent them from bumping into the bedframe.
A Very Marry Christmas - S8-E9
Revealing mistake: As the Hecks check out the neighborhood Christmas lights, the same house with the same decorations are seen through Sue's side of the car several times, showing the background scenes of the house and decorations were replayed.
Revealing mistake: While the ODIN agents are hallucinating, one can be seen looking into the barrel end of an MP40. If you follow him as the scene continues, the gun clips out of the sprite's visible area, and is almost entirely gone. (00:14:30)
Disneyland - S3-E22
Revealing mistake: The episode was filmed with real Disneyland visitors. One can constantly see them staring at the camera and/or Disneyland cast members standing with their backs to the actors, monitoring the crowd. This is especially noticeable when the family first arrives and when Mitchell and Cameron talk about Lily's leash: behind the actors, there are long white rope lines supervised by park staff to keep visitors at a distance. Several people can be seen filming the actors or taking pictures with their phones.
A Fistful of Paintballs - S2-E23
Revealing mistake: When Chang is getting blasted with the minigun, the safety pads under his shirt are very noticeable. (His shirt is extremely "puffed out" and you can see the outline of pads.) He's been tied up most of the episode and wasn't wearing pads earlier, so there's no way he was able to get pads on in the meantime.
Revealing mistake: When Ruxin's phone conversation with Kevin is overheard by Jenny, she starts an online chat with Ruxin. She sends her message, but almost as soon as she's sent it, for a fraction of a second we see "reply from Ruxin" appear on her screen, before the shot cuts to Ruxin actually seeing it and writing his reply.
Withdrawal - S2-E1
Revealing mistake: At 46:24, Peter and Mozzie are sat on a bench. Peter's paper has the exact same wording on back pages as well as the front page. (00:46:24)
Revealing mistake: After crashing the van into a lamp post, Terry looks into the wing mirror to spot the police. However, when he gets out of the van, you can see that the angle of the mirror would make it impossible to spot the police in relation to the van's position. (00:13:35)
Revealing mistake: In the scene where Rigby returns to the office and finds 15 messages on her answering machine, the second message is addressed to the name of the actor (Meaghan) rather than the character.
Revealing mistake: At the end of the episode when Leroy is trying to get a cat to come down off the house, it jumps at him and he gets into a "fight" with the cat going at his face. It is painfully obvious that the cat is just a stuffed toy and David Mann is just controlling it.
Episode #1.4 - S1-E4
Revealing mistake: Mrs. Weston's baby is obviously a doll.