Never had anything quite that bad...
But going to Internext Florida a few years back, I woke up about 2 seconds before we more or less free-fell for what seemed like 500ft or so.
It's absolutely amazing that *air* can give you such a bone-jarring crunch when the plan finally "catches". Had a few people that hit their heads on the ceiling and such, but luckily it wasn't too bad and no real injuries.
I had my seatbelt on
