Deliberate mistake: In the scene where Emmet drives to work, you get Emmet's view of the dashboard with his hand switching on the radio. The car at that moment has no front window, perhaps to make the car look bigger from the inside. (00:04:50)
Continuity mistake: After crashing the pig drawn car the characters fall onto a train car, the rail car directly behind the one they fall on is open and carrying barrels. In the very next scene none of the characters have moved, but there is now an enclosed train car there. (00:35:25)
Continuity mistake: During the chase scene in the Wild West, Bad Cop shoots a tracking device onto the back of Emmet's left leg. The tracking device disappears and re-appears at random until it is revealed during the attack by Bad Cop on Cloud Cuckoo Land.
Continuity mistake: In the real world, the dad seems to take apart a whole bunch of the son's creations and put them back to the way they were before. But in later shots, none of the son's creations seem to have changed.
Continuity mistake: When Emmet goes to buy overpriced coffee, Larry the Barista says "That's 37 bucks." In the background, you can see a woman in a Fabuland T-shirt is right next to the window. When Emmet replies "Awesome!" she has moved a (Lego) metre forward and someone is opposite her. (00:05:05)
Continuity mistake: While in the custody of Bad Cop, Emmet is chained by both hands to the melting apparatus. But when Wildstyle frees him, she only chops one chain to do so.