Every once in a blue moon a chain manufacturer overly heat treats the link plates causing them to be brittle. Sachs (pre SRAM) had this issue in the 1980s (or was it 1990s) for a short period. More frequently, but still uncommon, is a chain on a new bike that was assembled poorly, the pin isn't fully inserted into the side plate. This is the typical user install problem if a connecting link wasn't used. As mentioned first by Wilfred. Then more common is the hammer shifter who doesn't let off the pressure when shifting. This is usually what will create a twisted or bent tooth on either the cogs or rings. Then this "new" problem continues the chain breakage possibilities, even with reformed shifting technique. Andy.