I know at least two people who've had that happen. Unfortunately I don't know the brand of seatpost - one of my friends was on a Spec' Allez and probably had the stock seatpost. He didn't fall, but had to ride the next 40 miles standing up! I have no idea about the other one.
I'm still fiddling with my saddle often enough that it gets an incidental "inspection" at least twice a year. Fasteners - all fasteners - are one of those things that the service intervals ALWAYS call for periodic torque inspection, and NOBODY ever does it, not even when you take your vehicle to the shop for scheduled maintenance and it's supposed to be done. It's only when something like that happens that you wish you'd done it. Or when a bolt fell out of the triple tree on DH's motorcycle ...
Anyway, hope you feel better soon.