The Krakken - S1-E3
Continuity mistake: When The Krakken snatches the Cannery crate from the boat, you can see that the lower horizontal bar from the 'E' is suddenly missing, thus reading 'Cannfry'.
The Krakken - S1-E3
Continuity mistake: When Ben pretends to be sucked into the lake, and Gwen turns around to look after him saying "I'm not falling for it", she's missing her eyebrows for a second or two.
The Alliance - S1-E8
Continuity mistake: When Rojo attacks the police academy, there is a patrol car right behind her. But when Ben (as XLR8) arrives and slams into her, the car suddenly vanishes.
Continuity mistake: When the Tennyson's RV passes by Rapid City when it comes under attack by Vilgax, it's the first time the outskirts can be seen. But later, when the gas station explodes, the entire town is suddenly hidden behind a mountain range.
Continuity mistake: The number of rockets Sixsix has on his wrist launcher alternates between three and four, especially during the final combat scenes.
Continuity mistake: When the Marty-alien removes the dumpster as Ben watches him, there is a set of stairs leading down. But later, when Ben (as Wildmutt) and Gwen jump down the shaft, the stairs are gone and there is an iron ladder.
Continuity mistake: When Ben is upset because he is too short to go on the water slide, he is wearing a bathing suit. But in the shot where the Omnitrix deactivates and he is back to being a human, he is in his normal uniform.
Continuity mistake: When the three old people throw the car at Ben and Gwen, Ben doesn't have the Omnitrix on for a few seconds. (00:14:10)