I've got an Ultegra 6600 setup with FSA Gossamer cranks, all pretty
much new (roughly 1000 miles so far.) First chain was a KMC DX10 which
snapped a couple weeks ago. I'd hung the bike up wet and left it in a
basement for a few weeks, and by the time I picked it up spots of rust
were visible on the chain. I measured it for stretch but found none,
lubed and cleaned it, but it snapped while in a lowish gear easing
away from a stop light. I figured that I'd have to chalk it up to
rust.
I picked up a new Shimano 105 chain, removed extra links, used their
rivet to connect, checked it for stiff links, but that chain also
snapped after maybe 30 miles of easy use at one of the regular rivets
(not the installation one).
Naturally, I checked the rings and cogs for anything seriously askew
but couldn't find it. The derrailleur alignment looks pretty good.
It's a 53/39 and 12-23 setup so there's no crazy gearing, and I
maintain as straightish a chainline as reasonably possible. In all,
the drivetrain runs pretty smoothly.
I reconnected the 105 chain with a KMC 10sp connector and I'm crossing
my fingers.
Bad luck? Something else to look for? It's annoying having to ride so
gingerly.