Ouch!
So sorry for your misfortune and the damaged finish to your saddle, Ian. What a pity.
Glad you weren't hurt.
The bolt broke near where the threads meet the solid shaft. The last few turns of the die can sometimes create stress raisers and that is where they seem to go in my experience. Yes, overtightening could well have been a factor. I lube the clamp interface as well as the bolt threads, then reduce torque by about 10% compared to dry recommendations.
I don't seem to break saddle/seatpost bolts, instead I fracture saddle rails, causing me to gain some experience in transferring my saddle leathers to new railsets -- 3 on my own bikes so far, several others for friends. Brooks' chromed rails were sometimes hydrogen-embrittled during the chroming process; the black powdercoated replacements seem to be holding up well for me so far...along with using long layback seatposts so the clamps are in the middle of the rails instead of cantilevered from the forward end.
That said, I always take spare seat-binder bolts and spare bolts for the saddle-rail clamp. I can't afford a failure in the middle of nowhere, so a spare bolt or two added to the kit is reassuring and don't weigh much. I take extra bolts for my SPD cleats, too. They can be stored in the Nomad's rim-dynamo bracket on the fork.
Sorry for your misfortune, but grateful for your kindness in relating this cautionary tale.
All the best,
Dan.
