Deliberate mistake: Loch Ness is portrayed as a clear lake, as shown in the underwater scene. In real life, Loch Ness is so murky there is only six feet of visibility. Obviously deliberately done for the sake of the storyline.
Continuity mistake: When Fred gets to the North Pole and the Security elves take him down. Fred comes up and the snow on his face change between shots.
Revealing mistake: In the scene at the beginning of the film when the bus enters the village, you can see that the road signs are on the left side of the road (not facing the bus) and the continous line that separates the two ways is on the exit of the curve, not before the curve. This is because the action of the movie is set to happen in England but that scene was filmed in Romania, a country with right-hand traffic. (00:03:20)
Visible crew/equipment: When the ambulance is about to crash into the tanker, right before and during the crash a camera mounted (painted white) on top of the cab is seen.
Continuity mistake: When Sarah is on the beach in a gown and slippers, she takes the slippers off, then they are back on, then seen off, on the sand, again.
Continuity mistake: All wide shots of Hrothgar's hall show it in the outer ward of his fortress, about two hundred metres from the towers on the edge of the cliff, with open ground on three sides and an embankment on the fourth. But later several characters are able to step straight out of the hall onto a balcony that overhangs the cliffs and sea below.
Factual error: When the wraiths first invade the stockade, one of the officers orders his men to fire their "rifles". This is the 16th century. They're firing muskets.
Continuity mistake: When Vivian is walking to meet Aiden for their date, Vivian's dress changes from black to brown.
Continuity mistake: When the girl is looking up the fire engine in the oversized book, the little blonde boy in the red T-Shirt on the stool next to her changes the position of his head and hands between shots as she brings the book to the desk.
Continuity mistake: In the beginning of Part 3, Ryan Reynolds is on a deserted two-lane highway asking Hope Davis for help. She refuses to help, begins to turn and walk away down the double-yellow center lines of the road. In the next shot, we see her finishing the turn and walking on the edge of the road, by the white line (facing traffic). Immediately after, she is shown on the right side of the road, on the other white line (walking with traffic). (01:14:15)
Continuity mistake: When Frylock is watching New Jersey getting destroyed by the Insane-O-Flex, look for a building to the left through the Ferris Wheel. When Meatwad transforms into a building, the fires briefly stop, then turn on again.
Continuity mistake: Anxious to gift-wrap more pork, Angela scuttles by her uncle and tries to take over. He replies "Brown paper and tape", and she desists. This short exchange consists of three cuts where Christina Milian's hand goes up, is down but off the counter, and then down on the counter. (00:00:45)