I crashed Windows.
Back when I was doing QA for Packard Bell/NEC in the era of Pentium 200MHz & Windows 95, I figured out how to crash the system using the registry editor & MS Paint. Open the editor, copy a bunch of text, open paint, paste in the text, edit the text (it didn't matter how only that you made some change), copy the edited text from paint, paste it back into the registry, & tell it to save. The system would promptly shit itself since paint did something wonky to the text that the registry editor didn't like & the whole thing keeled over like the Titanic.
I notified my boss whom contacted the software folks whom contacted MS; we got a visit from a bunch of MS geeks later that week & they had me repeat the issue. The computer wasn't the only thing to shit itself that day. The MS geeks left in disbelief because "That's not possible!" except they now had proof that not only was it but easily done.
It was at that point my fellow QA team handed me & my partner matching t-shirts: white background, black classic bomb-with-burning-fuze logo & black lettering proclaiming "QA Bomb Squad - if you want it to pass don't hand it to me!" I loved that shirt. I think I wore it to death.
AC because if my old boss is reading this he'll come smack me for all the hell I put him through. :-D