In this picture of a 105 - 5700 chain:
http://www.competitivecyclist.com/im.../98508_i_1.jpg
you can see that each pin has a little "hole" in it's center.

One or two pins in the whole chain don't have that hole; they are flat.
A couple of rides before, I removed one of that flat pins so that I could properly clean the (very dirty) chain, and then reinstalled it. The chain failed at that flat pin.
It it matters, I accidentally removed that pin completely and it was hell to put it back... it was understandably weak.

When I fixed it on the roadside, I used the standard, partially removed pin from the joining link.


Another failure is waiting aroung the corner, I reckon...