mistakes you've felt you've made? what happened?

I made my boss hire a friend to be lead because he was good. When he became lead, he became very hierarchical. Just unreasonable. Mr I do it the right way had a Jenkins node running on a local laptop attached to our real build system. Wtf? Also, he didn't do much effort himself to produce some good code, people merged his code anyway and weren't critical bevause he was lead. It was my role, I shouldn't have give it up but I was burning out. In the end still did due frustration. I felt betrayed

I once exposed an endpoint that could be called to send any arbitrary email from our servers :^)

Giving the ceo of my current workplace the chance to prove that she’s not mentally deficient. I should have left a year ago or earlier.
It says more about my ability to recognize things than it says about her being a fucking idiot, so I am more mad at myself than at her.

...that crash in a certain retro racer on one of the handheld consoles.

Missed a certain very specific scenario, where you had to boot the game w/o any save data in main story, go straight to the level editor, create a level, save it, then attempt to play it.

I'm still beating myself for it.

This is but one example, as there were more of those not-so-obvious scenarios.