I avoid mushrooms if I can. They're slimy, they taste bad, they feel rubbery in my mouth, they even smell gross when you cook them. Just thinking about them has caused me to lose my appetite before.
I also hate olives. They seem to have a "heavy" taste that I always have trouble describing, since I don't know of anything else that tastes like them. Olive oil is fine, but I don't think there's anything special about it.
Even worse, though, are pickles. Their taste just contaminates everything they touch. It's never even as simple as removing pickles from your sandwich- the taste is still there! This one is especially odd for me since I love sour/tart foods and I enjoy some other pickled items (like pickled plum in Japanese onigiri, and pickled ginger, used with sushi.) There's just something about pickled cucumbers that I can't stand.
I'm a vegan now (I do not eat nor use any animal products, including meat, fish, shellfish, milk, eggs, honey, leather, etc.), but when I was a kid, I couldn't stand many types of meat. Meatloaf was a top offender. Anything full of fat really bothered me, too. When I was given steak, I usually ended up cutting half of it off and throwing it out since I thought they had too much fat, which made it really chewy and awkward (my parents were not happy about that!) I also always hated bacon. I can't, for the life of me, understand why there seems to be a sudden internet-wide unanimous vote that bacon is awesome. I can't even be in the house when somebody's cooking it without me gagging from the smell. As well, I can't stand the smell of cooking eggs. I don't even know what fried eggs tastes like, because ever since I was a kid, the smell of them cooking was enough to drive me away.
One more thing, I never could tolerate fish. My dad is an avid fisherman, so he'd often catch and cook the fish himself. I couldn't figure out how to cut it up on my plate, first of all. It always seemed to fall apart. It doesn't get any better in my mouth. But then I also had to taste it... blech!