Revealing mistake: During the big fight scene, you can tell easily that Frank is wearing knee pads (try looking when his pant leg is wet). Construction workers will often wear knee pads for work but most often they wear them on the outside of the pants to save the knees of their pants. (00:58:50)
Revealing mistake: As Reg fights with one of the baddies with the chainsaws, as the baddie take a swing with the chainsaw he misses and cuts into the wooden door. You can clearly see that a vertical cut has already been made in the wood though, to help the chainsaw get in and cut. (01:17:55)
Revealing mistake: Apparently filmed inside an old warehouse, heavy masonry walls can be observed during some scenes inside what is supposed to be a spaceship.
Revealing mistake: A wire can briefly be seen on MAC, as he does the flips in the air, in McDonalds, in the teddy suit. (01:02:55)
Revealing mistake: In the last scene of the film where the Reverend is holding up the glass jar with the mini-blob in it, you can see the mechanism being used to move the blob (look between the Reverend's fingers). (01:30:30)
Revealing mistake: When Michael and Spike are having a dance off, behind them the road continues. It is so obvious that the road is just a backdrop, along the road the texture and colours change at one particular point, also the sky.
Revealing mistake: None of the grenades Nick throws at the bar does any damage.
Revealing mistake: When the kid who breaks the egg throws it out the window, the egg hits another egg which starts rolling down. You can see the tracks on the grass making it easier for the egg to roll. (00:30:05)
Revealing mistake: In the scene when the aliens first enter Valerie's house, Mac and Zeebo find the fish tank and eat one fish each. The shot turns to Valerie's stunned reaction then back to the aliens, who are smiling widely, pleased with their stunt. If you look at Mac's teeth, you can see they are bright white up to his first molar, which is a give-away at his fake white fronts.
Revealing mistake: When Dave and Mike enter the area where the cocoons are stored, you can tell that all the rows above the main floor area, with the exception of the second rows near the camera position, are all matte paintings.
Revealing mistake: After Johnny Five falls out the window and whips out his hang-glider, as he turns right-side up and soars past the buildings, you can see that his body is transparent (an artifact of the optical matting process used).