I had a truck's rear u-joint go while I was following it once. The
center flew out and hit my windshield right between my eyes. Luckily,
it was spinning beneficially to the slant of the windshield, so my
F-150 glass only got a shearing scratch there, no puncture. Most of
it polished out. I was -really- lucky that day. A truck cross through
the forehead would have kinda hurt. I avoided the drive shaft which
the truck jumped over because I was in the next lane over by that
time. Talk about adrenaline shakes...

Reminded me of this video:

http://www.youtube.com/watch?v=rOhgMAOrp0c

As a kid I remember my dad coming home one day and telling us of a lost
driveshaft scare he's had... IIRC he said it came from the opposite side
of the freeway bouncing/flailing end over end through traffic, hopped
the center divider for another bounce or two before impaling itself
javelin style into an ice-plant covered embankment.

I've heard, (but don't know to be fact) that US Federal Motor Vehicle
Safety Standards now require vehicles contain and prevent driveshaft
separation in the event of joint failure.

Erik