An extremely unpleasant surprise when passing gas.
To quote the greatest philosopher of the modern age, Mr. Homer J Simpson, "Sweet Merciful Crap!"
Supplement from 05/31/2008 08:45pm:
I almost forgot; when made into a sandwich, it is an exceptional metaphor for life.
"Life is like a Turd sandwich: The more bread you have, the less **** you have to eat."
Mayo or Miracle Whip- you decide.