Continuity mistake: In the episode 'The 525 CrackerVac' the vacuum cleaner malfunctions (as usual) and Gromit challenges the device like in a duel. The scenery changes to orange like in a sunset and Gromit stands with his back towards the window. On the window sill you can see a cactus. Wallace is in the room near the door. The vacuum cleaner explodes and covers Wallace in ashes/dust. Wallace suddenly has the cactus on his head (also covered in ashes) and judging by the fact that there is a clean area behind Wallace on the wallpaper in the shape of Wallace with a cactus on his head he must have had the cactus on his head before the explosion, but we know he hadn't as the cactus was standing behind Gromit and Gromit stays clean after the explosion, so the cactus couldn't have moved. How did the cactus end up on Wallace's head?
Continuity mistake: When the thumbs are in the car driving to the woods, in some shots their thumbs are out of their shirt a lot, but in other shots, they're in there all the way.
Deliberate mistake: When all the thumbs are at the wall saying "I'm the king of the globe" and "I'm a dentist" you cannot see their legs, so that saved some costume work.
Factual error: The raiding party approaches the French ship in small boats without using muffled oars which was a must to approach silently. Oars were wrapped with cloth to deaden the sound.