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I haven't eaten meat but I've definitely accidentally eaten gelatin. It's awkward. But you never promised to be perfect. It's ok to make mistakes. It's happens to people. It doesn't make you a bad vegetarian/vegan.
I did this just a few months ago.. Took a bite into something that had bacon in it. I spit it out but the thought and taste in my mouth made my physically ill. I cried too, the guilt got the best of me...
Recently at a friend's house I was served a bean casserole which was supposed to contain ground beef. She said she would leave some without for me, which I appreciated. While I was eating this delicious dish, I asked for the recipe. She told me the various kinds of beans and then came to pork and beans. Wait, what?
The point is that it was not maliciously done, and I don't fault her for it.
I had no adverse effects, and realize it was an honest error.
I would look at it like this - if you didn't buy the meal then you didn't fund the meat industry and therefore there is nothing to feel guilty about. Even if you did accidentally buy it, then write it off as a mistake. Adding more of your own stress to this world is not going to help the animals.
I did the same a long time ago. I felt horrid after!