When an evil pink villain is resurrected after millions of years lying dormant, transforms several times, repeatedly gets stronger through absorption, then destroys Earth and many other planets... there's a fair bit of blame to share around the Dragon Ball Z heroes. As the strongest fighters and central characters, the Saiyans receive plenty of that blame -- Vegeta turning Majin and fighting Goku, Goku underestimating Buu, Gotenks wasting precious fusion time, or Gohan not finishing the job when he has the chance.
But there's an argument to be made that perhaps the most culpable character of all is in fact a godly entity: the Supreme Kai. Debuting with the impression of a highly mysterious but extremely powerful being, Supreme Kai's shortcomings are laid bare over the course of the Buu Saga. Despite his vast knowledge acquired over eons of time, he is constantly surprised and left helpless as Buu ravages the Earth and the universe. Let's take a look at Supreme Kai's biggest mistakes and assess where he could -- and probably should -- have done better.
Supreme Kai Didn't Warn The Z Fighters About Dabura's Powers
A critical mistake comes early on when the Z fighters arrive at the location of Babidi's spaceship. When the group spots Babidi and his henchmen, Supreme Kai and Kibito immediately recognize Dabura as the King of the Demon World. They also knew about his unique abilities beforehand, as Supreme Kai explains that Dabura's spit can turn anyone into stone -- after Piccolo and Krillin have already been hit by it. Had he shared that information sooner, the Z fighters surely would have been more on guard and perhaps even Kibito's life could've been saved.
Supreme Kai Underestimated The Saiyans
Underestimation is a common theme throughout the Buu story arc, and the Babidi Saga provided several examples of Supreme Kai's misjudgments in that area. He is unnerved by fighters like Pui Pui and Yakon -- both of whom are dispatched easily by Vegeta and Goku -- and suggests that the group should all fight together as a team. It revealed not only a lack of knowledge about their abilities but also a lack of understanding of how Saiyans' minds work when approaching a battle.
The Saiyans repeatedly brush off his advice in dealing with their adversaries -- damaging enough to Kai's illustrious reputation -- and it all comes to a head when he fails to stop Goku fighting Majin Vegeta. The fight that, despite Goku's good intentions, ultimately gifts Babidi the energy needed for Buu's resurrection.
Supreme Kai Misinterpreted the Legend of the Z Sword
Supreme Kai came to Earth in the belief that Gohan was the "chosen one" who could master the legendary Z Sword, an ancient weapon embedded on top of a plateau on the Sacred World of the Kais. As he understood it, the legend goes that whoever can remove the sword will gain access to legendary new powers, enough to defeat Majin Buu. All of the former Kai tried and failed in this endeavor. Gohan does in fact remove the incredibly heavy sword and gains some strength from training with it, but it's not nearly enough to take down Super Buu.
Quite simply, Supreme Kai gets it wrong but lucks out thanks to Goku showing up on the Kai planet and doing typical Goku things. While helping Gohan train with the Z Sword, he throws a giant block of katchin -- the strongest metal in the universe -- which accidentally breaks the sword and frees Old Kai, whose powers help unlock Gohan's full fighting ability.
Supreme Kai Didn't Warn Gohan About Buu's Absorption Technique
Unfortunately, Supreme Kai didn't take full advantage of the gift he'd been given thanks to Old Kai's appearance. A fully-empowered Gohan returns to Earth and wipes the floor with Buu, but sees the tables turned when Buu absorbs Super Saiyan 3 Gotenks and Piccolo. Gohan and the boys' overconfidence is entirely to blame for that fiasco, right?
It turns out, thanks to a flashback showing how the other Supreme Kai lost their lives, Buu had in fact absorbed two of them into his own being: South Supreme Kai and then Grand Supreme Kai. This meant that the current Supreme Kai knew about Buu's absorption ability all along and spent all that time with Gohan, yet never thought to warn him before he left. It was a costly error, as Gohan surely wouldn't have allowed Goten and Trunks to fuse again if he knew they could be absorbed.
Supreme Kai ultimately ends the Buu Saga portrayed as a rather weak universal ruler who needed Goku and Vegeta (and Mr. Satan) to bail him out after a string of consequential misjudgments. A running theme through his mistakes is the failure to impart all the knowledge he'd acquired over his lifetime and his previous experience of fighting Buu. Had he been more aware of previous events on Earth and the Saiyans' true abilities, he could have aided them more effectively by sharing everything he knew about Majin Buu right from the start.