I managed to crack a seatpost on an old BMX just from riding it for months with barely an inch of the shaft actually inside the seat tube. it just snapped clean off while I was riding, I managed not to wipe out but riding the 4 miles home with no seat was a drag. I remember removing the bit of post still lodged in the seat tube was impossible, so we had to hammer it further down into the frame, yikes.
that was an awesome set up though, a laid-back post with the deep angle, sitting a good 17" off the frame ... if I hung back on the seat enough I could ride that 20" with near full extension