We have a friend with an egg allergy, dairy allergy, and MMA. They're vegan as well by choice but it's made me very careful. I've briefly upset a few vegans and vegetarians by questioning them in a bit more depth. It's happened much less over time as I've refined how I clarify and ask.
I'm always amused when someone says vegetarian. Okay, what type? Can I serve you cheese? Eggs? Fish? One of my coworkers is also vegetarian but eats chicken...which was new to me.
I think it's religious? I didn't have a chance to dig in at the time.
My primary concern was getting all the different vegetarians to identify what type they were. As an example one of them was lacto-vegetarian but was fine if eggs were in something (bread). I was laser focused on by feeding everyone with appropriate diets and allergy considerations.
Honestly thinking back I wish I'd had the time for more detail. We had someone with a lethal fish allergy and another with a crustacean allergy so never asked about sea food. Wonder if it was a religious and/or cultural thing to not eat pigs or cows.