The Spanish Inquisition - S2-E2
Revealing mistake: At the very end when The Spanish Inquisition bursts into the courtroom, you can see that Biggles is not played by Terry Jones (as he is playing the accused judge in the court sketch).
The Way We Almost Weren't - S2-E23
Revealing mistake: Closer to the end of the episode, when Lilly and Oliver are lying on the floor glued to the chairs, you can easily see black strips on their backs attaching them to the chairs.
Revealing mistake: When Georgia walks into Billy's office naked, a triangle-lamp will cover her "naked" breasts. Look closely, for at one point you can see the white, strapless bra.
Revealing mistake: During the dinner scene where Dave is thinking about Arsenio Hall eating cheese, Arsenio starts complaining about how good the cheese is. After punching one guy, he goes up to another and slaps him in the face. When he slaps him you can see he misses the guy's face by a good inch or so.
Mark Hamill - S4-E16
Revealing mistake: When Luke Skywalker is introducing his 'cousin' Mark Hamill at the end, you can tell it's a split screen by the fact that Luke looks right past him, even once he's on stage. (00:25:10)
Revealing mistake: The purple dress that Micky and Davy bring for Mrs. Weefers to wear is too big for her, showing that the dress was made specifically for Micky to wear.
Revealing mistake: Patty Duke's stand-in is visible behind Cathy when the family is in the front hall welcoming their aunt.
This Male Chauvinist Piggy Went to Market - S3-E1
Revealing mistake: When Laurie goes to flip Goose over her shoulder, it's obvious that a stuntman is actually flipped; he has much shorter hair than the actor portraying Goose. (00:11:45)
Revealing mistake: In the scene at the beginning when Scarlett is signing autographs after hers and Gunnar's small concert, you can see her signing a poster with a marker that doesn't leave any ink on the poster. (00:08:15)
Adam Driver/Chris Stapleton - S41-E10
Revealing mistake: Star Wars Undercover Boss: Starkiller Base: After Tim insults Kylo Ren as being fat, 'Matt' gets angry and Force chokes him before throwing him across the room into the vending machine. In the wide shot, you can see the rip line tied to Tim's back that pulls him into the vending machine. (00:03:50)
Revealing mistake: When Marco is telling Ellie about getting study candles, the power goes out, but behind them the electrical fan is still running.