Sounds like human error or equipment failure to me. Serious, no one is going to come and pour fresh water in your tank. I am with Wasp, it would be bleach, a hammer or Godfather style, a horses head ( just a joke, don't want to offend all those horse people).
He probably did a water change and forgot to put the salt in the mix. Didn't Alois just about turn his reef into a freshwater tank?
Hey Cracker, would your system not be prone to this if that plumbing fitting fails?