Continuity mistake: The name on Travis' helmet switches between 'Ryer' to 'Travis' several times throughout the movie.
Visible crew/equipment: When Jack and Jackie are waiting in the car for Dr. Becker, Jack is looking in the side mirror. When Jack sees him he opens the door, a camera is visible in the side mirror.
Continuity mistake: After Train transports everyone across the narrow bridge over the lava, he slouches down to the ground in relief. In the next shot he is standing straight, looking normal.
Other mistake: Michelle Rodriguez's accent changes several times, not only throughout the movie, but sometimes within the same scene.
Other mistake: When Dorthy is trying to audition for the Muppet's the man says they are gone and the Muppet's bus pulls up. If you look when the bus is pulling up there is no one in it, even the driver seat is empty. Then a few seconds after the bus stops Kermit and Miss. Piggy come walking out.
Continuity mistake: In the scene just after Andre has dragged Angela from the river, towards the beginning of the film, the two engage in a lengthy conversation. As the camera moves between close-ups of Angela's face, and shots of Andre, where Angela is standing in the background, her hair keeps changing from being off her face to covering her face.
Other mistake: Throughout the movie large footprints of heffalumps are shown. However, when the heffalumps bound around, they never create any footprints of any kind.
Continuity mistake: When Beowulf enters the water cave and seeing Grendel's body, the initial shot has his right arm sitting next to him (most likely placed there after his mother's took it from the Meat Hall). When Beowulf leans over to touch Grendel's chest, the arm is gone.