Revealing mistake: When the lawn mower the heroes send off as a diversion crashes you can see a wire pulling it. (01:01:30)
Revealing mistake: When the graboid starts pulling Jim underground, his wife hands him a board to keep him above ground. You can see the thick board bend rather easily while his wife hands it to him. Also when Jim is sucked under ground it bends quite easily as well, as if it were styrofoam.