Considering that we are talking about that specific chain and that he admittedly may have messed up one of the links I immediately figured it would be the chain, as I've heard (and felt) quite a few of them click and pop right where the fouled link is.

My refusal to believe that people still don't know how to properly tighten a cog or lockring made me overlook it. I suppose it could be a possibility, although a cog slipping usually doesn't make that much noise.