Back in 1999 I somehow managed to make my project du jour (a text editor) crash Windows 95 so hard that after rebooting it popped up a message telling you to reinstall Windows. :D

I'm definitely guilty of writing some sick memory-trashjng bugs when I was first getting started as an intern. The best one had my team lead and a staff engineer puzzling over what the hell was going on. The second best one had someone hunting down my email and messaging me months after the internship ended just to cuss me out haha. Perhaps it is better I don't work in C anymore.