I volunteered to help setup a middle school dance, and got paired on four tasks with the most beautiful girl in school. After working together for two weeks, and working the dance night together, as the night was winding down, she asked me to dance, and I said no.
I had never danced before--in public that is--and didn't want to embarrass myself. She seemed disappointed after asking three times and still getting a no answer. At that moment, whatever friendship we built in the last two weeks was gone. Later, towards the end of the dance, I saw her dancing with someone else...someone who would later become her boyfriend.
I messed up badly. This was the stupidest thing I have ever done. It not only affected our relationship, but put me in a "no confidence" cave, to this day, I have never really left.