Back in the early noughties I had an interview for the new job. A couple days before the interview I've visited that company's website. There was search input. Of cause I've entered some hacky things into it. And after several attempts I hacked it. The site was down in an infinite loop.
Two days later I told interviewer about the bug and what I did to reproduce it. He was surprised and checked the website. It was still down the same way.
I was totally ashamed. I was supposed to report that problem somehow.
BTW I got the job:)