Goofs

The size of the giants is not consistent throughout the film, along with some of the miniature props they handle.

When the giants shrink back to normal size, the amount of fabric they clutch to preserve their modesty is less than it should be.

When Mike and Nancy are confronted by the giant spider at Nancy and Genius' house, Mike kills it by kicking loose a pipe and when the basement is flooded he throws a broken light socket into the water and electrocutes it. However, in later scenes, especially the one where Fred and his gang break in to steal the goo, which takes place a few minutes after Mike and Nancy leave, there is no trace of either the flood or the burned carcass of the spider.

The first moments when we see the giant sized dog in the movie, the now large sized pet is no longer wearing his dog collar around his neck like he was previously prior to his growth. Most likely due to his having grown out of it. No point after that as a giant sized dog is he seen wearing a dog collar. Seconds after the dog is shrunk down by the antidote at the end of the film, now at his regular size he has his collar around his neck again.

Georgette is seen right before the teens eat the food, but is not shown growing and is not shown again until the final scenes of the movie.

When the girl on the motorcycle is driving through and around Fred's legs with the rope, his legs are far too far apart and do not taper in at all (to where his groin would be above), revealing the fact that they are simply two badly placed props.

When the kids take the goo and begin to grow, their clothes start to tear and they become naked. However, the rubber band holding Jean's pony tail in place does not come undone as she grows.

Visible wires attached to the giant ducks in the nightclub making them jump into the air.

After the fallen Fred is tied to the back of the red convertible, it loses its bumper and rear chassis prematurely as crew members yank it off before the car even so much as lurches forward.

After the giants return to normal size, there is a shot of a man on the courthouse steps, and the shadow of the camera is visible in the bottom left hand corner.

When Mike (Tommy Kirk) walks off the courthouse steps to confront Fred (Beau Bridges), the shadow of a camera can be seen in the lower left corner for a few seconds.