Deliberate mistake: At graduation, after Troy is handed the diploma, in the overhead shot the graduates on the left and right stand-up from their seats, then turn and walk towards each other. All their seats simply disappear between shots. Obviously done deliberately for the dance sequence, but a mistake nonetheless.
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: 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: The priest that Ray kills doesn't stagger out of the confessional booth toward the little boy until he's already been shot twice, yet when Ray sees the boy he accidentally shot, he is still kneeling in the prayer position and therefore did not move or react whatsoever to the first few gunshots only a few feet away.
Deliberate mistake: When Penny is loaded onto the ambulance at the end, her gurney is rolled straight in. This is wrong, the paramedics will collapse the wheels and slide the gurney into locks to hold it still during the drive. It was done this way since Mittens and Rhino were hiding in the collapsible section.
Deliberate mistake: Director Adam McKay states on the commentary that they went to the island Catalina for location searches and realised that he did not really like the island. Instead, on the coast and during several scenes, the island of Catalina can be seen in the background. He chose not to have it digitally edited out as a joke.
Deliberate mistake: When Zohan and Phantom are seen on the Palestine beach competing about which one is the tougher, they let themselves get bitten by a piranha to prove their point. While piranha are by legend very vicious fish, its presence here is a two-fold mistake, for a) piranhas are found in South America, and b) they are fresh water fish. (I know that this is for pure humour, but the writers should have picked a rabid crab or something; that would be more appropriate for the setting.).
Deliberate mistake: When Seth Green and Lance pull up to Ian and Felicia, the car being pulled by the carriage should roll a little bit after the carriage stops moving, but instead it stops on a dime when it is only being pulled by rope, and nothing else to stop it with when the carriage stops. (00:42:00)
Deliberate mistake: When Ami's school friend is killed, a short sword is stabbed into the top of her head. Less than a half-second later, we see her face is totally coated in blood, and she gurgles even more blood before dying. There simply has not been enough time for such a volume of blood to have dripped down her face.