To the original question;
It depends what is non-vegan about the dish, and how the dish's non-veganness came about. I always ask about dairy, as I'm lactose intolerant. If it has dairy in it, even after every single specification I've made, it has to be sent back. This has only happened twice that I can remember. In one instance, there was a slice of cheese on a veggie burger, so I just took it off (it was not in any way stuck to anything) and gave it to my boyfriend. If I had sent it back, the whole thing would have gone in the bin, and I hate wasting food of any kind.
Similarly, if the dish contained meat, I'd send it back. This has never happened in the UK (where labelling and understanding of terms is usually ok, and if it appears like it's not, I don't eat there), but it happened on a recent holiday. I thought I had ordered a tomato pasta dish. Language barrier caused some confusion, and I ended up with something covered in bacon. I didn't eat it, but I didn't send it back because I was worried about what I'd get in exchange. They noticed I hadn't touched it, and I wasn't charged.
However, if the dish contained egg, and I had just assumed it didn't, then I'd probably eat it. That, or I'd give it to someone else and order something that was vegan. Again, it's the wasting food thing. This has only happened twice - once at a sushi bar, where I asked if the noodles in the noodle soup contained egg. They didn't, so I ordered it. There was half a boiled egg floating in the soup, so I gave that to my friend and ate it anyway. I just try and learn from these mistakes, and ask better questions in the future.
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