Whether it's in the war room or the bedroom, anime characters can make some spectacularly bad decisions. Sometimes, these poor choices are seemingly inconsequential, like when Erika Shinohara of Wolf Girl & Black Prince lies her way into a terrible relationship. In other cases, the repercussions are apparent from a mile away.
In the DBZ anime, Vegeta is too proud to stop Cell from achieving his perfect form, and his hubris ends up indirectly killing many of the show's heroes. Bad anime decisions can be frustrating to watch, but they also reveal characters' all too human flaws and inspire complex plot lines. If a fictional character's mistakes are big enough to make you angry in real life, the story is usually doing something right.
Letting your opponent get a significant powerup is never a great idea, and Vegeta of Dragon Ball Z learns this lesson the hard way. Every time Cell absorbs another android, he unlocks an entirely new form, which comes with a serious power increase.
Just before he transforms into Perfect Cell, the villain says Vegeta ought to be challenging himself with stronger opponents. Vegeta's pride gets the better of him, and he allows Cell to absorb Android 18, thus unlocking his final form. Once this happens, Vegeta no longer stands a chance, and the Saiyan is quickly taken down along with Krillin and Future Trunks. Although Cell is eventually defeated, this transformation makes him an unnecessarily dangerous opponent.
Even though the show's mantra is "Gotta Catch 'Em All," Ash Ketchum seems dead-set on releasing his most powerful Pokémon for basically no reason. From a narrative perspective, this makes it way easier to showcase new Pokémon, but it's super frustrating for Pokémon fans, as Ash routinely dumps powerful allies with little practical justification.
His constant self-sabotage makes it hard to take him seriously as a protagonist, as his stated goal of being "the very best" is rarely reflected in his actions.
When the Black Knights discover Lelouch's true identity, they instantly turn against him. Unfortunately, this is perhaps the biggest mistake ever made in Code Geass, as Lelouch is the only reason the organization made any headway in their rebellion against the repressive Brittainian government.
With him gone, they have no way to defend themselves against their enemies, and are easily defeated.
Light Yagami makes a lot of mistakes — the first being his decision to start using the Death Note — but once the story really gets going, his biggest slip-up comes when he falls for L's well-laid trap. In an effort to test Light's skills, the great detective forces a death row inmate named Lind L. Tailor to appear on TV, claiming to be L.
Light kills the man using the Death Note, giving L valuable insight into Kira's location, as the TV broadcast was only available in the Kanto region of Japan. Light plays right into L's hand, and although it happens early on, this misstep contributes to his eventual downfall at the end of Death Note.