Revealing mistake: When "Pop" dies, you can tell he is still breathing, especially if you fast forward the scene. He is just doing it very slowly, but you can see his chest move up and down.
Revealing mistake: When Catfish Hunter jumps out of the water towards the beginning of the movie, you can see that the splash of him landing in the water is actually in front of where he actually lands.
Revealing mistake: When Catfish Hunter swims away at the end, you can see the shadow of a crew-member or some sort of device operating the puppet fish, under the water.
Revealing mistake: When the kid in the store keeps knocking down the bottles and cans, you can see he barely grazes them, and yet they all fly off the shelves.