Many, but one stands out:
In grad school I had a professor who really liked me; we got along great - went out to bars, had parties at his house, just hung out and were very good friends (He was pretty young, maybe 35).
This guy was also very connected (had been a lawyer before decideing to go into education) and secured jobs for a lot of people. Anyway, to make a long story short: He got frustrated about on-the-side jobs that some of us grad students were doing. He felt we should be concentrating on our work as grad students (and to be fair - some of the jobs conficted in a VERY slight way with some of our responsiblities).
So at the beginnig of my second year he called us out (in front of undergrads) and gave a pretty mean speech that ended up with the line "If you want to do jobs on your own, go live in public housing. Don't waste my time".
That line burned through me like a laser. I had spent the previous 3 years finishing up my undergrad degree supporting myself with whatever money I could find. I lived in the basement of an old ladie's house like a homeless hobo.
Now I was actually making enough money to buy a pizza occasionally (our grad assistant pay was pathetic) and this guy was telling me to "go live in public housing"? I was furious.
I refused to talk to him in any social way after that. I only talked to him only to respond to school related things in a cold way. He would call me; I never returned his calls.
He would ask me what happened, and I would say "nothing". After a few months he stopped trying to interact with me, and I finished my schooling and left without ever clearing anything up.
Now it's ten years later. I've realized that time deson't stop, it just keeps on going.
I've also realized that I let ONE remark made in ONE lecture on ONE night completely ruin what could have been the best friendship of my life. I should have just let it go.
I definitely regret that.