Continuity mistake: Season 4, episode 10 "Ben 10 vs. The Negative 10, Part 2": When the Forever King makes his announcement "Nothing will stop this little home-coming", he stands outside the Mount Rushmore base on bare soil. But in the scene where Max reviews the recording of this speech, all of a sudden we find the Forever King standing on a neatly tiled floor.
Sam: Part 1 - S1-E7
Other mistake: When the Ranger first yells "SPD emergency," a piece of Jack's Morpher breaks off.
Avatar Day - S2-E5
Continuity mistake: When the village leader switches the wheel of punishment to Community Service, it is on the space next to Boiled in Oil. However, all of the other shots of the wheel have another section in between these two.
Factual error: Haloclines are referred to throughout, and depicted occasionally as some sort of underwater tornadoes. Haloclines are in fact just salinity gradients, and look nothing like they do on the show.