Continuity mistake: The vehicle used by the person delivering newspapers (an Austin Mini I believe) has, in one scene, an "A" license plate and in the other scenes it has a "B" plate.
Continuity mistake: When Lucy has purchased an orange tree and catches a taxi, its number is different than the one that drops her off at Barry's shop. Could be put down to her catching two different taxi's - however when she leaves the shop and climbs into another taxi, this one has the same number as the very first one she climbed into.
Factual error: Whenever Steve Coogan is driving his old brown car during the 70s and 80s, you can look out of his windows and see cars that weren't made then, even some with current registration plates on, either driving past, or parked up in driveways, or on the side of the road.
Continuity mistake: Augustine's hair has a life of its own. It becomes messed up a few shots before she fights with Gaby, then is back to its proper position during the piano shot of her song, then it's back to messy - but in a different way than before - for the standing up part of the song.
Factual error: The phrase "Si, se puede" is translated in the movie to mean "Yes, we can". However, "Si, se puede" literally translates to "It's Possible". (When it is mentioned earlier in the film that "Si, se puede" means "Yes, I can", that is also incorrect.) "Si, nos podemos" is incorrect. It would have to be "Si podemos" to be grammatically correct. Yet, in the United States, the phrase is colloquially translated to "Yes, we can", so it is probable that the writers and directors made the decision consciously, especially considering that the director is of Spanish descent.
Continuity mistake: When the convict comes back to say sorry for killing Earl's dad after the shootout, watch every shot of Tinker in this scene. In one of them, you can see Tinker's back teeth shiny white and clean, while his front teeth are his usual rotten yellow colour. Obviously, the make up artists never applied the fake yellow teeth to the back molars.
Continuity mistake: When the team has their first game against the old guys, the first shot by the lead is wrong. Watch carefully and you will notice that the direction of the rotation of the stones changes about three times from clockwise to counter-clockwise during the same shot.
Continuity mistake: In the taxi scene, where the 'club kids' are discussing who's on Gleemonex, the taxi passes Adventureland Superstore twice in about 10 seconds.
Continuity mistake: When Jerry-Lee is at the breeder's home, he starts to eat some flowers. In the first shot he picks up one stem of flowers and pulls them out of the vase. Camera cuts and he starts to pull them out again. (00:21:10)
Continuity mistake: When Jerry hits the cab its windscreen cracks. The subsequent shots of the cab (when Katie helps him up, and when they drive off) show no sign of a crack. (01:17:35 - 01:18:55)
Revealing mistake: Hercules walks out at night and suddenly encounters Megara in the forest. Directly afterwards you see them walking off in the distance. The wall behind them has some torches on it, but apparently these were added in post production: One of them shines right through their bodies as they walk past. (00:47:00)
Plot hole: Throughout the movie, the flies are vulnerable to light (direct sunlight and even lightbulbs) to the point of incinerating in a split second. Except... they are not; in several sequences they fly to and from bodies and without even taking a direct path (look at all the action happening in front of the house, by the cars, in broad daylight).
Revealing mistake: When Pally and Charlotte are in bed, Charlotte's pillowcase barely goes above the headboard, yet its shadow casts on the wall is much larger and higher, and has a triangular shape. (00:29:13)
Continuity mistake: During the chase in chariots (2 Romans against Astérix), when the palace is attacked, in one shot, we can see there is only one Roman soldier on his chariot instead two. (01:21:55)