Shel from greenandprofitable.com shared this story:
Decades ago when I was working nights, I was once tricked by a formerly veg housemate, who left me a note saying “have some leftover pea soup.” I wondered while eating what veggie substitute she’d found that tasted so much like ham. She had totally forgotten I’m vegetarian!
But in restaurants, I won’t accept meat in something labeled vegetarian. In your situation, I would have politely called the waitperson over, said that you ordered the soup because it was listed as vegetarian on the menu, pointed out the meat, and asked politely what they could bring you instead. And of course, you shouldn’t be charged for the unacceptable item. I consider it a vital service to other vegetarians to educate people that e.g., baked beans with bacon or cheese with ham or even veggies cooked in chicken stock is not vegetarian. (I will make an exception if I’m a guest in someone’s home in a foreign country, and they’ve labored over some treat that has stray bits of meat in it.)
I do find it necessary to ask frequently since menus aren’t always clear. But it’s much easier now than it was 38 years ago when I stopped eating meat.