Mushrooms...anything with mushrooms...dirty, horrible, slimy fungus
I didn't eat mushrooms for a decade because, as a kid, I gorged on them so much I puked my guts out one day and from then on associated "mushrooms" with "vomiting".
On a dare, I had the entree from a "Veggie Burger with Barbecue Sauce" MRE. I don't know what I was expecting, but it had the texture of wet cardboard, the taste of salty cardboard, and had a (I cannot make this up) little broccoli floret in the mashed together faux-meat patty.
Heck, mostly anything in MREs can be classified under this, but that veggie burger is what stands out the most.