McDonalds.
But seriously now. Worst thing I've had for breakfast was when I was little, I had a bowl of Rice Crispies, and thought a bowl of salt was the bowl of sugar, so I sprinkled salt on top, and the first bite was a nightmare. I guess it doesn't really count though since I didn't really bother eating it.
Weirdest breakfasts:
Pizza(Almost everyone has done that at least once)
Carrot muffin with cream cheese icing spread on top
Chocolate cake shavings in a bowl with milk poured over it
A double decker grilled cheese(I was feeling inventive. I was not disappointed)
Pierogis cooked in cheddar cheese/cream of mushroom soup with smokies chopped up in it
There's probably more, can't think of any right now.
EDIT: A little off topic, but the worst thing I have ever bitten is a burrito that had been stored in my friend's locker for over 3 weeks. I don't think it's possible for anything to taste worse than that burrito did without being capable of melting your mouth. Although, given enough time, I wouldn't be surprised if that burrito could have done just that.