Revealing mistake: Near the end, right when "little wonders" starts, Lewis takes a moment to look at his future family, Lucille, Frannie, etc. When there's a closeup on Lewis, his memory scanner reveals a reflection of no one in the gym, except for Lewis, even though people are supposed to be standing in front of him.
Revealing mistake: Doomsday drops Superman once he hears a little girl crying and walks up to her to try and smash her. As he is walking up, it shows a close up of the crying girl and she is wearing a pink shirt with denim overalls on. But look closely and you can see a mistake in how she was drawn. The straps of the overalls are incomplete and stop a few centimeters away from the buttons on them that are supposed to hold the straps in place, and the buttons are between the straps and her overalls, with a gap between the buttons and the front part as well. It's not connected at all and should be falling off her. (00:25:15)
Revealing mistake: When the Level 3 Sentry is firing up into the air for no apparent reason, it fires some rockets with a clear defined smoke trail behind them and disappear off screen. Then the shot jump cuts to further away and suddenly the smoke appears again, but the end of the smoke trails are solid and crisp indicating the animators used the smoke animation as if the rockets had just been fired from that spot... in the middle of the sky. (00:01:10)
Revealing mistake: When everyone is cheering to Bender, you can see Fry on the right hand side of the screen wave to him twice. The second time he cheers, his hands move from the table to suddenly waving at Bender without any animation. (01:19:50)
Revealing mistake: Meatwad and Shake get booted off the computer by Frylock so he can find instructions to the Insane-O-Flex. Watch the computer monitor. It rumbles, but the logo on it stays still.
Revealing mistake: When Grandpa is being rolled out of the church, everyone is looking at him. As this happens, look closely at the Squeaky Voiced Teen sitting next to Hans Moleman on the right hand side of the church. As he turns his head around you'll see his eyes disappear as he stops turning his head. (00:06:05)