Ack, it's the worst feeling, isn't it?
I was at my Aunt's house a few days before Christmas: she had made some dinner, and I wasn't expecting it at all, but she let me know that the lasagna she made was vegetarian. Halfway through, though, I noticed something chewy, salty, and rubbery. I forked around, and low and behold, there were a few minuscule rogue sausage pieces in the lasagna! I put my fork down and felt guilty about it the whole night--I just wanted to go home and clean myself out or something. All I could think about was how I had a piece of dead animal inside me. I nearly had an anxiety attack.
I didn't question her about it, but my guess is she probably started making a lasagna with meat and then remembered I was coming over, so scraped it out, leaving some tiny pieces behind. I'd much rather she just made the damn thing as she intended and simply let me know: I'm perfectly happy either fasting or eating any veg suitable fare around the house.
It was a pretty rough experience, but it made me even stronger in my vegetarianism than ever (or at least made me realize how strong it was): I now know I'll be veg for life.