I guess I've been lucky with food so far. As far as I can remember, only ever had to deal with a hair in a curry, which I spotted long before consumption so all was well.
With drink, I've had horrible ales sneak up on me at beerfests, but nothing truly repellent.
If I take a small liberty with your criteria, and add deliberate (albeit drunken) modifications, I can give a tale of the foulest concoction I've ever imbibed.
It was the end result of a drinking game (ring of fire, if you wanted to know), and so we now had a mystery concoction of mixed lagers, nothing too bad yet. My friend, upon drawing the card which required him to drink this pint, took it upon himself to... spice things up (almost quite literally in fact). Soon, the drink had mustard, vinegar, curry powder, worcester sauce and anything else he could find in the kitchen within it. The rest of us decided to do a shot each of this beverage (now a murky, yellow-green colour) to see what it was like. To say it was foul is about as blatant an understatement as you are ever likely to see. I've never tasted anything so criminally vile as that one drink. How I kept it down is beyond me...
Bizarrely, the effect on the guy who had to do the pint wasn't sickness (though that did follow eventually) as much as rage at the sheer offense to his tastebuds it presented, and so he went about punching the walls and doors of the house with reckless abandon...
That was a good night.
