a friend, an experienced touring cyclist, has suffered a serious injury after the bolt holding the seat post clamp sheared. Now....this is the kind of thing that always happens to someone else, riding some other kind of bike, but - I've seen a carbon post fail at the top and leave a jagged stump pointing up - I've had an aluminium post fail, and leave a jagged stump - the seat post clamp bolt that failed was steel - I've seen an alloy saddle rail break And, speaking from personal experience, having a seat post fail is a bit like having a chain go - it takes a while for you to work out what's happening - and a lot of bad stuff can happen while you're working it out. Having said that the carbon seat post got floppy before it went entirely, as did my aluminium seat post, so there was some kind of warning there. So.......if you feel your saddle shifting - stand up on the pedals and come to a halt. And just check those bolts from time to time to see if they're cracked. There's no guarantees, but you just might save yourself some pain and suffering.