There are three instances, which I can recall in 10+ years of vegetarianism, so I suppose that's not so bad. Each of them had an interesting, somewhat
positive side effect as well.
The first was ages ago, when I went to a local coffeehouse in Michigan. I asked about the stock in the veg. soup..if it was meat based (I wasn't vegan then). I was told it was vegetarian. When I tasted it, I became pretty sure that the soup contained chicken broth. I asked one of the staff about it, and it was in fact chicken stock. The waitress/staff then came back apologized, and we had a long talk about whether it was hard to be veg., etc. Later (like a few months), she went vegetarian!
Many years later, I went to this bitchin' festival in a town near where my parents live, and through either a direct lie or complete incompetence by a worker, I ate about 1/2 a meat hot dog. I was very upset. I went over and was sitting down, chilling, and saw my (very omnivorous) father go over and confront the person who'd served me the meat. I'd never realized until then, that he actually respected my decision.
The third incident was just a couple of weeks ago when (this time, due to stupidity on my part), I ate some crackers that had prawn in them. I was again upset, but through the events that followed that evening (which are long and personal, heh), I got a bit of a greater understanding of how much my love really cares for me.