Not meat but I have dropped a bollock or two with dairy/eggs.
First, I went a month or so before I realised Quorn had egg and milk in it
(well, I thought it was a f***in' mushroom didn't I?!)
Then I was in a Thai restaurant last year and I'd spent so long making sure the dish didn't have fish sauce in it I neglected to notice the fact that the fried strips of tofu arrived in batter. I was still rabbiting to people about how Thai people put fish sauce in so many dishes as I ate half a dozen of them...
While I was in Thailand I once went to a small restaurant and had a long conversation, mostly in mime, with the owner about which foods were veggie. (I wasn't a vegan at the time.) The basic menu was printed in English as well as Thai, so I wasn't too worried about the fact that no-one there had more than a word or two of English. I got on really well with them; I was nice to their kids, they were nice to me - even to the extent of putting extra pork in my Special Vegetarian Rice.