Plot hole: When the mutated sea gull attacks the Tennyson's' sailing boat, it brings down the sail. But then the piece from under which Ben and Gwen emerge suddenly has a convenient wooden lattice fixed to it, whereas in reality it should have none.
Ben 10 vs. The Negative 10: Part 2 - S4-E9
Plot hole: 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?