Revealing mistake: Carl, Eddie, and Steve are out ice fishing. At one point Carl falls through the "ice" and you can clearly see that the "ice" is made of some sort of styrofoam. It looks like the stuff used for packaging. The snow looks just as fake.
The Simpsons Christmas Special: Simpsons Roasting on an Open Fire - S1-E1
Revealing mistake: After Homer brings Santa's Little Helper home, Homer and Marge hug, and if you look behind them, the background is upside down. There is a picture hanging upside down, and there is a lamp which seems to be hanging from the ceiling. Then in the next shot, the background is normal. (00:21:40)
The Library - S1-E3
Revealing mistake: In the extra 'Library' scene, the book that Bean is given already has the loose pages before he has been at them with the knife. (At one point the pages get pushed forward a little bit, meaning that they're not fastened into the book.)
Star-Crossed - June 15, 1972 - S1-E3
Revealing mistake: Sam stares at himself in the mirror and takes out his wallet to determine who he's leaped into. But the "reflection" in the mirror lowers the wallet and looks back up before Sam does. (00:05:20)
The Million Dollar Bond Robbery - S3-E3
Revealing mistake: All the Liberty Bonds loaded in the bag of Mr. Ridgeway have the exact same serial number. (00:20:00)
Revealing mistake: When the first fight starts, the Bloober disappears into thin air. (00:04:52)
The Ringer - S1-E1
Revealing mistake: When Link calls down to Zelda from the tower, you can see a fault in the drawing as Zelda moves. There is a low wall around her on the balcony, with flowers and vines on it. As she moves, you can see some of her dress pass right through the wall and vines towards the lower right. (00:01:50)
Save the Max - S2-E3
Revealing mistake: When they are reading the reviews in the newspaper, look at the back page. It's the sports section. Next when Jessie comes back to Zack and tells him what Mr Belding was hiding, she says the paper is from 1968. But look at the back of that paper also. They are using the exact same paper. The reviews and the 1968 paper are the exact same.
Beauty Rest - S4-E5
Revealing mistake: Kathy Ireland is supposed to be dead but watch carefully and you'll see her chest move when Minnie Driver's doing CPR.
Shanpû no hangeki! Hissatsu shiatsuken wa mimo kokomo ubau - S1-E16
Revealing mistake: In his close-up when he grins at Ranma after his request to help him figure out what happened to Akane, Ryoga is drawn really obviously with the hair going under his eyebrows. (00:15:00)
Revealing mistake: When Arran is pushing his gran at high speed in her wheelchair through the care home corridors, it is obvious that his gran is a dummy.
Revealing mistake: When they arrived at Montana, the Captain said that this is where we will stop and build a ranch. There are two straight depressions in the ground converging together that would not have been there since this was virgin land.