Long time ago, during my first few months at Microsoft while working in the Exchange team I found a bug. After much deliberation I took myself to the developer's office. I was scared - those days I was scared about everything at work I think. I politely knocked on the door and with my "I am sorry but..." signature line entered the room. He was a very smart guy, listened to me patiently then very elaborately spent the next ten minutes explaining how my bug was not a bug. I was sure it was an issue but he was the developer and he was so eloquent and so confident that two minutes into the talk I believed him. On the 11th minute he realized a flaw in his explanation.
"Hmm, it is a bug. Forget all I told you so far."
I can't remember what he said after that but those lines really wiped out any lack of confidence I had. For ten minutes I had believed him. He had been speculating so confidently that I believed him. I doubted myself and really had believed him.
I shed my sorry self and walked out a different person. If you ask me what my best bug at Microsoft was I have to say that one. It grew me a backbone and was the foundation many a bug stood their ground on.