It's true, everyone has had this experience. While it's absolutely disgusting, it's better to try to get over it and move on.
I've had quite a few of these experiences. Found beef in my vegetables at Cheesecake Factory (they messed it up the first time and added meat. Instead of making it again, they just picked the beef out). I've had soup at a restaurant before where the waiter swears it's vegetarian (claims to have checked) and then I go back, ask again, and another person says it's definately not. When my boyfriend and I first started dating, his Mom didn't know anything about vegetarianism. She's the sweetest thing and makes me special meals now (something my own Mom wouldn't do in 100 years). Since she's still not exposed to it much, she doesn't know what to look for on labels (and she doesn't read them all). She didn't realize that chicken broth wasn't vegetarian. She bought stuffing with chicken fat before (I didn't eat it though). The other night, she made a salad and after dinner, I found the recipe on the counter and there was Worcestershire sauce in it. Luckily, it was only 1/4 tsp and it served about 10 people. I didn't end up sick or anything. I think omnis just don't understand how to read a label. Us veggies are pros!