another_jim wrote:Dried coffee is the same as instant coffee -- so some of it will redissolves and give an excessively bitter taste.

However, it is kind of hard to believe that a layer of dried coffee that survives water backflushing, detergent backflushing, blank shots, overnight soaks in detergent, etc. is going to redissolve to any nonhomeopathic extent. I suspect that even in that first disgusting photo, the coffee layer has polymerized to such an extent that essentially none of it is dissolving into subsequent shots.

Anyone worried that contamination (from any source) is affecting their shots should draw water through the coffeeless pf, let it cool, and taste it.

You have a good point. I was thinking only of people who do not flush after the shot, getting the remainders in the the subsequent one. But that also presupposes they do not flush before the shot either. That could be true in some cafes; but it's unlikely among enthusiasts.

Of course, judging by some of the other discussions, we have amateurs here quite capable of discerning even the most homeopathic levels of contaminents.