It could accidently slip, do a little flip, go into into your throat, bleeting like a goat, suffocating you till your face turns blue, and you'd choke to death instead of dying with the flu :) I just made that up.
I was dumb enough to do this twice...(the epitome of young and stupid). First off, everyone assumed I was some sort of turbo slut because of it. They get infected easily and you have to reconcile whether having thousands of dollars of dental work from busting your teeth is really worth having a piece of metal jammed through your tongue. They can also affect your speech permanently if they put it in the wrong place.