Mine kind of spans both worlds - something that was meant to be eaten, but not by itself, particularly in concentration.
When I was a kid I came under the mistaken impression that the reason why icing tasted good was because of the red food coloring my mom put in it (she liked to make cakes and cupcakes with pink icing for me). Soo - why not just go to the source? One day I took the bottle of food coloring and took a swig directly out of it.
OMFG.
To this day I don’t use food coloring at all.
Noooooooooooooo! I still haven’t gotten over the last time you posted this story!
hurk
hurghk
Oh my god.
I haven’t eaten stinky tofu but have smelled it from roadside carts on many occasions. I will eat just about anything, but have never had the courage to try stinky tofu. Open sewers smell better. I’m not joking.
So, probably the worst thing would be … Hmmm… I can’t think of anything that was supposed to be eaten actually being that horrid. Perhaps Dave’s Private Reserve Insanity Sauce comes close. I like it IN foods, but you honestly can’t try it straight up, like I did, before running to the bathroom and puking.
As a teenager I ate a Gainsborough dog food patty on a dare. It literally tasted like shit. I can still taste it.
ETA: And count me in with the cigarette-butt-in-the-beer-can crowd too. Blech!
Does mine count? Its kind of a combination between the two.
Do you know those Atkins Advantage chocolate shakes? They come in a cardboard juicebox like container.
Did you know they could go bad? I didn’t, and once took a giant gulp of one that had… Worst thing I have ever experienced and I could not get the taste out of my mouth. Brushed my teeth multiple times, and even drank three beers later, but I could still taste it.
When I was young, at our parties, we used recently-emptied beer bottles as ashtrays. In some drunken haze, I took this bottle I thought was mine and emptied it in a hearty gulp.
Stale, lukewarm Irish beer and cigarette ash do not taste good. At all.
That’s kind of like taking a nice big swig from a milk jug that’s seen better days.
I’ve never actually done that, but on occasion I’ve hurled at the mere thought of emptying one down the sink.
1a. I’ve eaten all kinds of crazy things intentionally (grasshoppers and cicadas perhaps the craziest), but for as much as I love all kinds of sushi, I still can’t abide by sea urchin. I won’t exactly retch it, but for the life of me, I don’t know how anyone could enjoy it, regardless of country. My guess is that it’s the alleged aphrodisiac properties that give all the motivation-- the stuff tastes like a Kyoto bar bet gone horribly, horribly wrong.
1b. Sator bean (it’s in some Thai food). This isn’t a taste issue-- the taste isn’t anything to write home about, just another green bean-- but the after effect is extraordinary. Think asparagus, but exponentially stronger and lasting two full days. I think my urine smelled like it came from a dead person, it’s rank beyond belief.
Genuine maple syrup on my waffles.
I don’t do coffee. I had diet Pepsi.
I wonder if rat poison tastes like that.
Never had hakarl, but surströmming is by far the foulest substance I’ve ever eaten (and I enjoy pretty much all the foodstuffs mentioned in this thread so far.) I like fermented foods, I like pickled foods, I like herring, I had no idea what I was getting into when I mailed ordered a tin of surstromming (which is basically fermented herring, though some may call it “rotting”)one day when I was bored. I ate a small piece or two on the stoop and accidentally spilled a little bit of the fermenting liquid onto the steps. Within about a minute, the entire area was covered with a solid mass of flies, I shit not. The fish smelled like a cross between a dog’s ass, rotten eggs, vinegar, decomposing fish, finished with a slight whiff of household cleaner. And, despite protestations on some internet sites to the contrary, it does not taste better than it smells. It tastes exactly like it smells.
It’s the only food I’ve never been able to take more than a couple bites of. The worst part of it was I drank down some beer, so for hours later I was burping up this horrible, acrid odor and taste.
Mmm, purple soap gum. I’ll have to get a pack next time I see it. 
Genuine maple syrup is the most disgusting thing you’ve ever tasted? So strange. ETA: Oh, you mean the combo of maple syrup and Diet Pepsi!
My worst was my introduction to mussels - I took one bite, and spit it out immediately and cleaned my tongue with my napkin. I’d never had mussels before, but apparently they aren’t supposed to taste like death. As you can imagine, I’m not a mussel fan at this point.
A thing that isn’t supposed to be eaten? Hmm. I can’t really think of any right now.
I bought a bottle of Mars Milk (chocolate milkshake, essentially) at a service station once, and opened it just as I was driving back onto the motorway. Took a big swig and… it was off. Really off. I had coagulated lumps of caramelly, chocolatey CHEESE in my mouth, and I had just merged at 70mph into heavy traffic. This was not a good thing.
You really have to wonder about a food that’s only “ready to be eaten” when the can starts bulging outwards. No wonder they suggest it should be opened underwater.
For me:
Abovementioned surströmming, probably, although a close contender is raw oysters, not because of the flavor but because it reminds me of swallowing a huge loogie and I just can’t take the mental image.
I was changing my youngest little brother’s diaper once and yawned at just the wrong moment. Got hit in the mouth with a stream. Luckily the changing table was positioned right above the bathtub so the spit and vomit was easy to rinse away.
She wouldn’t even touch it, but I was determined to get my money’s worth. I’ll eat pretty much anything, and I HATE to waste food, but I couldn’t take more than a couple of bites. Just awful. Not spoiled or rank, just really, really bad. Ugh.
2a) I’ve also done both the tobacco-spit and cigarette butt thing.
Intended never to be eaten:
Industrial polymer used as a flocculant in water and wastewater treatment. I hate working with that fucking stuff. It’s incredibly tenacious, slimy, sticky, white, semen look-alike nastiness that swells into slimy, sticky, semi-solid snot-like grossness on contract with water. It can be a nightmare trying to clean large spills.
Once I wasn’t careful enough and a bit clung to a uniform sleeve. When I went to get a glass of water it brushed off on the rim. Even that tiny little bit instantly coated my entire mouth with with a persistent layer of INCREDIBLY ASTRINGENT slime. Not pleasant.
ETA: #2 would be warm tobacco spit swilled from a coke can my brother had been using. shudder
cow tongue. I was on a plane flight from Omsk to St. Petersburg, Russia and this large, unwashed guy from Murmansk had the sat next to mine. He smelled horrible, but was trying to be friendly and offered me a piece of cow tongue that he brought aboard for his dinner. In exchange, I gave him a piece of my Bounty Bar (like an Almond Joy but no nut, or a Mounds with milk chocolate) that I had managed to find at the hideous Omsk Airport. Worst trade ever! I spent much of the rest of the flight in the plane’s bathroom.
pancake syrup that had mold growing (and floating) on it.
I was on a fishing trip once and reached for a Coke can. What I didn’t realize that one of the guys was using it as a spittoon. Still gives me the shivers!
Dried Peyote buttons were really bad (but well worth it!), however the worst was Asparagus Juice, a popular box drink in Hong Kong.
Who thought of that stuff?
Intended to be eaten: Lychee fruit. I don’t know if it’s supposed to taste like an eyeball that’s been marinated in that stinky perfume that little old grannies wear or if the one I got was off, but it was the most horrible alleged food I’d ever put in my mouth, including omeboshi and lutefisk on the wrong side of a day too old.
Not intended to be eaten: a big swig of drained motor oil that was handily poured into a 2-liter bottle of Pepsi to be taken off to be properly disposed of. I didn’t actually swallow, and then voluntarily used soap (dishwashing liquid) in my mouth to get it all out. Horrible, horrible awful taste.
I guess this isn’t disgusting so much as painful, I once popped an entire dollop of wasabi in my mouth, not knowing what it was. My friends stared at me, and one of them went “…did you just put that entire thing in your mouth?” THEN I felt it. On the plus side, my sinuses were clearer than they’d ever been for like, the rest of the month.
As for most disgusting, I once tried gunpowder tea with maple syrup in it. Never again. That shit was NASTY.
Also, I live in Minnesota and have not tried lutefisk. Go me.