Yes, explosion.
Carboy was in the kitchen sink with a big blow-off tube going to the other side of the sink. In the middle of the night there was a very, very loud BOOOM! The huge blow-off tube we had stuck on there had managed to get clogged, and the carboy had exploded and sent glass shrapnel in every direction, taking out some light fixtures, and covering every surface that had a view of the carboy in beer. We even found glass stuck into the ceiling of the adjoining dining room.
It is for this reason that I a) don't use glass carboys and b) don't trust blow-off tubes.