My favorite yogurt makes me sick. I've still eaten it three times after the first time when I found out that it was both my favorite and the one that would try to kill me. (Not as serious as death, just bad)
Took me forever to realize raw onions were a problem since they aren't if they are well cooked and some kinds of onions are fine (red and green). And it's not like I ever sit down to a plate of onion.
I never knew onions affected me because I hate onions? But I had some blooming onions at some point and my stomach did not function right for like three days after.