Which gears were you in when it broke? Front & rear? What terrain were you on? eg. Smooth uphill, rocky uphill, short steep power climb etc. or just flat technical terrain etc. Perhaps your gear selection is what's at fault here, there is alot more strain on the chain in various gear combinations, it's just a bad habbit then.. and could be unlearnt, possibly a combination of the two, where the chain was not 100% correctly *broken* and spliced and also removing the stiffness from the spliced link... This should indicate if you're properly fixing the shimano chain. Most of the vetran Mountain bikers use the SRAM chains, It's only ever happened to me with a shimano chain years ago, but noting the *new* improved system for inserting the shimano chains it's become a rareity... The sram system is soo much easier and reliable.. The only incident i've heard was a guy that managed to *unhook* his power link, by pedaling backwards and having chain suck... and i've been riding for 15yrs. Take a powerlink or two with you as a spare.
