I was also under the assumption that a gearbox would give you lots of warning before it finally let go - noise, vibration, etc. This report seems to indicate that isn't always the case.
40 years ago, my dad was nearly killed when the gearbox of his Yamaha seized without warning. A part had cracked and jammed itself in between some of the rotating bits, immediately stopping the rear wheel.
This may only be remotely comparable to a helicopter gearbox failure - then again, i don't believe there are many people who have experienced one of those and lived to tell about it.