Deliberate mistake: When Phil wakes up at 6 AM, it's already bright outside. At 6 AM in February, it would still be pitch-dark.
Deliberate mistake: During the "breaking up" scene with Jim and Michelle when they're inside the cabin, her hair is crimpy, but when she walks outside and gets into her car, her hair is straight. This is because Alyson Hannigan filmed this particular scene during a break in the filming of a "Buffy the Vampire Slayer" episode. (01:15:40)
Deliberate mistake: Bosley is a school crossing guard trying to help the Angels. The children behind them say they are gonna be late for school. Of course they're going to be late because there is not a school within 10 miles of where they are standing. There's trees and what looks like a huge mountainside on the other side of the street. You can see this more visibly as the car that is about to run over Bosley is approaching. Done for a gag, but still ridiculous. (01:09:05)
Deliberate mistake: When the Trunchbull is drinking from the glass with the newt in it, you can easily see that she doesn't actually drink anything, as the water level stays exactly the same. Logical, as the newt is real, but it's still very obvious because she drinks for about 15 seconds. (00:46:55)
Deliberate mistake: Towards the end of the movie fish are seen to swim down in order to submerge the net in which they have been caught. Although the attempt seems logical, it actually needs power against the net to push it down. None of the fish seem to touch the net at the bottom however and if it is the whole mass of fish achieving it, then the bottom fish should have been squashed against the net which (again) isn't the case. 'What brings the net down?' is the question here. (Done deliberately by Disney/Pixar to be visually aesthetically pleasing.)
Deliberate mistake: When Boo laughs repeatedly while playing hide-n-seek with Sully in the men's room, nothing happens to the electricity - no surges as with all other times she laughs. [In the audio commentary by the directors, they explained that it would've been distracting if an energy surge occurred every time Boo laughed, so they simply didn't address the issue. A reasonable course of action, but still a mistake.] (00:37:30)
Deliberate mistake: When Austin and Felicity cross the street when they are shopping, you can see green hills behind them, very much like Hollywood and not 1969 London. (Deliberate reference to an earlier joke)
Deliberate mistake: In the first scene Velma gets sprayed by the hose and slips but gets her foot caught on the chain; hanging upside down, but her skirt doesn't go "up". Deliberate to maintain the "family movie" tone. (00:02:40)
Deliberate mistake: In the scene where the bad guy is attached to the missile and fired, in the split second before it hits the helicopter you can see below it a building called SUN BANK, but the letters are backwards. A close look reveals the likely reason - to the right of the reversed building is one which looks just like the building to the LEFT of the Sun Bank building behind the harrier a few seconds earlier. Most likely they couldn't film the chopper from the right side because there was a building there (the one the camera flies through), so they filmed it from the other side (the same side they'd filmed the harrier from), catching the "Sun Bank" behind, then had to flip it to have everything facing the right way. (02:06:55)
Deliberate mistake: At the end fight scene, when they are in the barn one of the men falls crashes through the wall when they get hit, spilling the fight to the outside. Unless the barn's walls were made of cheap plywood, the wall would not have given away so easily. Obviously done for dramatic purposes.
Deliberate mistake: The morning after the flyers are spread, Buster is sleeping in a drawer. When his alarm clock goes off, he hits his head at the top of the drawer, but later on, he can be seen standing up in it, easily visible when he is putting his coat on. The drawer obviously became wider for him to stand up in it.
Deliberate mistake: During the fire Marge runs back into the house and goes to the kitchen cabinet that has a "keepsake cabinet" nameplate and a keyhole, in order to remove the wedding video, but the nameplate and keyhole didn't exist before. (00:33:35)
Deliberate mistake: When the Evo crashes into the tanks it jumps into them at height but there was nothing there to make the car jump. (01:17:25)
Deliberate mistake: After the game has entered the real world, and Floop and Minion are there to "save the day", Carmen and Juni throws seven pairs of glasses to them. But afterwords, we see there are only four pairs of glasses on Minion and one pair on Floop, which totals five. (01:11:10)
Deliberate mistake: When the Looney Tunes are playing basketball against the aliens, if you look in the stands the same cartoon characters are in different places at the same time. This happens a lot.
Deliberate mistake: Manny refers to the pirate rabbit as the Easter Bunny, even though Easter never existed in prehistoric times. (00:33:55)
Suggested correction: And mammoths didn't hang out with sloths and smilodons either. It's creative license.
Deliberate mistake: During the scene of the Spartans moving out, they sing "I Will Survive." Sonio is clearly singing the song alone, because his lips are singing the song before the music or anyone else is singing.
Deliberate mistake: As "Fake Dolly" runs away from Gracie, it is rather obvious in the shot facing Dolly as she runs down the stairs and then when she turns to look behind her, that it really is a fake Dolly running. (00:48:20)
Deliberate mistake: When Max and 99 are in the air, they can be seen talking to each other. This is not possible (as proven on Mythbusters) even if you were yelling at the top of your lungs. The wind would make any noise unrecognizable, yet we hear them talking to each other just fine.
Deliberate mistake: There is a shot of the huge Yogurt statue that is in slow motion. You can see that the smoke from the statue is rising slower than it normally would. This was obviously done to make the shot longer for Mel Brooks' voice over to fit.