What are the worst experiences you made and dumbest things you did while developing an application?

Tbh, I usually inform myself and use tooling which is considered good. But even so, I made some really strange experiences, especially with debuggers...

What are some of your snippets which just refuse to work with your tooling or are so very stupid, that you would rewrite them this instant, if only that program would still be relevant? Let's learn from mistakes :D