Gohan being arrogant again is dumb because him acting like that is what caused his father to die because of his overconfidence, and he clearly didn't learn from what happened the last time he was arrogant.
Vegeta letting himself get possessed is dumb because he's putting the whole world at stake just because he can't handle the fact that his rival's more powerful.
Goku not finishing off Fat Buu is still a big mistake. Goku himself knows about the mistake and says he did it because he wanted Goten and Trunks to handle him. And that's not really a smart thing for Goku to be doing.
I think Anew has already explained why the Videl one is dumb
Super Buu absorbing Vegito isn't smart because Vegito could just rescue the others and weaken him from the inside.... and that's exactly what he did. Not really a smart move for Super Buu considering him doing that ironically makes him weaker.
It's indeed smart for Vegito in that his plan worked, it's dumb for Super Buu because Vegito wouldn't willingly let himself get absorbed for no other reason.