Krillin’s was especially dumb since he didn’t have blind arrogance as an excuse and 18 simply being brough back to life by the Dragon Balls after Cell is dealt with. Forgot the earliest dumb decision. Trunks giving the good guys a several year head start of what’s to come. And the good guys choosing to spend those years doing nothing but training rather than go on a manhunt for Gero before he could make any of his Androids. Even Trunks only goes back in time and rather than deal with Gero himself, settles for a warning before going back to his timeline. Goku gets extra points for not even bothering to take the heart medicine to save his life and it costing him the fight against 19 as a result