I heard about this phenomenon at a shop in Harvard Sq. where a filter problem led to some salty spro. The other day, I got to taste this first hand.
At first, I was lost on why the espresso was coming out so off at this one shop. Acrid and bile sours. Just horrible compared to the lab results.
The problem was not immediately obvious but after a series of tests and diagnosis issues, we finally found out what had happened. The water softener leaked into the espresso machine and brackish water was making for some nasty shots. What other damage may follow from this remains to be seen but at least we are aware of what happened.
Salty. If you ever heard the bit about putting a dash of salt into a bitter cup of coffee, it doesn't do good things for espresso. All the roast note vanished and sours became quite prominent.
The problem is being resolved but that was a weird experience trying to find out what was going on. You don't forget something like that.