Stupidity: The characters wish for the resurrection of just one person (Roshi) rather than everyone killed by Piccolo as would be natural (and as they do in anime and manga). Okay, Piccolo in this movie sucks at killing and resurrecting Muten covers 50% of the total body count, but they can't know that.
Stupidity: Goku defeats the henchwoman of Piccolo in the lava pit quite easily; she looks human. He is quite content to just push her off-camera with a smack, and nobody cares about trying to talk to her, figure out Piccolo's location and plan, or anything like that. Everyone just forgets about her, and it turns out she even eavesdrops on the lot before leaving. Someone would so casual about meeting and defeating the mysterious enemy only in a movie where you know the script already. (00:46:40)
Stupidity: The bullies run their car straight into someone they think is a total wimp (Goku) at full speed, crushing his bike. It's not a dystopian future, had Goku been a normal person they would have straight up murdered / crippled him for no reason at all. They are portrayed as a group of jocks, not homicidal maniacs committing roadkill on their way to school. It has also to be noted that Goku was parking his bike in a car parking spot, where nobody would just put a bicycle - but it is needed by the plot so that the bullies can casually crush his property. (00:06:35)