Stupidity: Having become the leader of the universe's most stealthy fighters, he promptly leads them in a Custer-esque noisy frontal charge into a force with known weapon and technology superiority. Zero effort to utilize ANY of their amazing stealth, just have them slaughtered so badly that the planet has to sacrifice animals to save them from extinction.
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: Rescuing Manfred and Diego from the giant carnivorous plant, Buck could have cut both wires simultaneously to avoid that unnecessary horrible potential outcome of cutting the wrong one. The wires were stretchable enough for them to have been held together to then get cut at the same time. (00:37:40)
Suggested correction: Cutting both wires simultaneously would probably cause a different effect. Besides, Buck isn't very intelligent.