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.
Continuity mistake: Season 4, episode 10 "Ben 10 vs. The Negative 10, Part 2": As we could see in first season's final episode, "Secrets", Roosevelt's head on Mt. Rushmore was totally destroyed, and in most subsequent episodes - including this one's first part - there is a sizable hole in its place. But after Ben asks whether the plumbers' hologram technology can replicate 'big presidents', and the scene changes to the demolished presidents' heads, the crown of Roosevelt's head suddenly joins the others on the mountainside.
Plot hole: 4-9 "Ben 10 vs. the Negative 10 (Part 1)": Rojo (from first season's "The Alliance") makes her second appearance in the series in a suit of powered armor. However, in "The Alliance" she had been turned into a cyborg by Vilgax' drone and thus looked radically different than she does here. So how can Max say: "How did she get back her alien tech?" when her equipment and weaponry was totally different in their first encounter?
Continuity mistake: Season 4, episode 9 "Ben 10 vs. The Negative 10, Part 1": When Charmcaster taunts Gwen about not having "taken off the training wheels", she moves her head upward. Note that the lower part of her ponytail falling over her backside does not move as the upper part does, but remains fixed in place.