For a few years following the end of college, I went back to eating meat. This came about because I took a trip to China my senior year with marching band, and the vegetarian meals they served were, for the most part, lacking in edibility and balance. So while I was there I gradually added in a few things, and then when I got home I couldn't stop. It was sort of like an addiction.
A year or two into this, we had moved to NYC and went to my brother's apartment for the 4th of July. We ate hot dogs. The amount wasn't really any more than normal, but it was more meat-heavy than my meals usually were. It seemed fine at the time.
Then later that night I started a stomach ache. It was so bad I could barely move, and I was doubled over in pain. Thinking it was my appendix, I had my father take me to the ER.
After laying there all night, hooked up to IVs, drinking that weird-tasting solution and getting all the tests, it was not appendicitis. The only logical explanation was that it was really severe indigestion.
It didn't strike me until a long while later that it was likely a terrible reaction to the meaty dinner I'd eaten. Sure, it could be a coincidence, but to me it's a sign that after so many years as a vegetarian, my body can't handle that kind of thing.
If you're one of those people who thinks it's funny to sneak animal products into the meals of someone who has chosen a vegetarian or vegan lifestyle, think about this. You are potentially making them very sick. If you do that, you're simply not a good person.