Electronic shrapnel flying through the room as the software explodes in a huge flash of electrons!
Seriesly,
If you're running windows, you could bork your installation to the point where it would have to be uninstalled and reinstalled from scratch. For Unix users, most likely the worst you could do would be to screw up your personal configuration files to the point that you'd have to hand-edit your prefs.js file, or others.
A couple of years ago, I had an early beta of firefox (before it was called firefox actually) that corrupted my config and toasted my booksmarks.html file. Fortunately, I regularly back up important things like that, so it didn't hurt too bad, but for those who don't, I'd imagine that losing your bookmarks would be a major bummer. My bookmark file goes back to netscape 1.x days, so if I lost it completely, I'd be really bummed.
"Electronic shrapnel flying through the room as the software explodes in a huge flash of electrons!"
Now, see, I'm just the type who would try it for this reason.