In my mind, I've answered this question before in the post "What's the weirdest thing you ever ate?"
But in the spirit of answerage, I'll try to make a differentiation here.
The worst thing I ever ate that was actually supposed to be food was whipped cream flavored with marmite and cucumber.
That was a joke.
The problem is that I don't remember the worst thing I ever ate. I really enjoyed my grandmother's cooking, and my mother's, and I don't recall going to someone's house and being so disgusted that I mouth yarfed my food back onto my plate. It's well documented that I think the cucumber is a douche in salad form but I don't recall the first time I ever ate it.
I do recall being penetrated once by a cucumber, and even though that may explain a few things about my hatred of the vegetable and my love of having things in my butt that are squishy, I wouldn't go so far as to claim I ate it with my butt.
Under-seasoned food in general pisses me off. If something has potential but doesn't arrive at it because someone was too light on the salt and pepper, I get a little angry. Mashed potatoes are a good example. I've had some pretty bland mashed potatoes in the past.
Mashed potatoes is also a pretty bland answer, isn't it?
Chili. I'm not a huge chili fan, but I can make the shit out of it. When I see an aborted concoction of tomatoes, kidney beans and beef with a half-teaspoon of generic chili powder, I want to murder on behalf of all people with face holes. That is not chili. That is under-seasoned beany goulash without the paprika. And no, you can't make chili (at least not with those ingredients) by letting it simmer for half an hour. What do you think you're accomplishing? Letting all the bean flavor out? Softening the ground meat? (You are cooking the rawness out of the tomatoes, which is good, but shut up.)
Short Answer: Squid ink jelly donut.
Note: Don't know why I'm so retarded, but I just realized that the actual question was 'what the worst thing you accidentally ate?' Don't know how I managed to forget what the question was as I was writing it, but whatever. And by the way, the worst thing I accidentally ate was your ability to use the letter S properly. Guess we're both a little retarded.