It depends on the circumstances - and their mindset. If the meat is not properly and safely cooked - leaving some raw bacteria (which may make anyone sick, but a vegan/vegetarian would be expecially vulnerable to due to the lack of currently built-up resistance) then it can be much more serious. That is the biggest risk factor I'm aware of, and the reason why vegans/vegetarians are encouraged *not* to try reintroducing meat into their diet whilst eating out at a restaurant or an event with a "pot luck" style catering, since there is more chance of food poisoning at such places generally. Well, that's the advice I've heard over and over again, anyway.
I found a good article that covers most of what I've been taught :
http://sciencenordic.com/does-meat-make-vegetarians-ill
"?A person might have a little problem if he or she started right out on a huge steak. Their body might not have sufficient levels of the right enzymes. Along the same lines it can be hard to digest any large meal after a long fast or period of starvation,? he says.
You might also have this problem if you start to eat other kinds of foods that you haven?t eaten in a long time.
The lonely geek who does nothing but heat up frozen pizzas day in and day out might experience some intestinal discomfort if he suddenly starts eating a lot of fruit and vegetables."
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Being careful is a precaution more than anything else. But given how fraught re-introducing meat can be (even for partial vegetarians, the psychological element shouldn't be discounted), it's important to be informed in order to plan accordingly.