Revealing mistake: When Valentine grabs Mindy off the pogo stick, you can tell it's been stuck into the ground so the actress can stand on it and be ready for Kevin Bacon to grab her (as opposed to Kevin Bacon grabbing her while she's bouncing). After she's grabbed, you can see the pogo stick shake back and forth without falling over.
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.