Some shots of Merrie dancing reveal that Horsey is absent from her chest (they do not have a doll hanging off her shoulder straps for the effect). Also, when they show close-ups of the boy with the cowboy hat hanging from her chest, the other giant teens are absent.
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.
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.