Some fish are infected with the parasite kudoa, also called soft spot, which causes fish to liquefy post-mortem. It is nearly undetectable in some fish, that is, until you cook it, so you can't really blame your fishmonger for selling it. The parasites' presence has nothing to do with freshness, nor is it harmful to humans if digested. It happens often in black cod, tuna, salmon, halibut, and can sometimes happen in other species such as mahi, hake, sole, and flounder. Usually when this happens at the restaurant level the infected fish is set aside and returned the next day. The case can be that one fish is infected out of ten, with that fish being returned while the other nine are perfectly fine to prepare. When it happens at the consumer level, especially in cases where the person is unfamiliar with the parasite, markets can receive some very angry phone calls. I understand the disappointment, but the real culprit in these cases is most often a hidden defect and not a careless fishmonger.
Harvesting, buying, and preparing wild seafood is not as formulated as other mass produced proteins like chicken, beef, or pork. There are things that can happen to the animals, whether due to human neglect or a natural occurrence, that can't always be identified before the proteins make it to your plate. When you purchase wild proteins, take into consideration that you are purchasing an animal that has lived a life of fighting and struggling against the dangers and elements of nature, not a life of being hand-fed in a controlled environment. I recommend buying your seafood from reputable places and developing good relationships with your fishmongers. From time to time there might be something that goes wrong with your wild seafood purchase and, in those cases, the store should always refund your money. Just remember though, depending on what the case is, it's not always a sign of a negligent fish market when something goes wrong. Sometimes no matter how hard the supplier has tried, Mother Nature will prove that when buying wild, there are no guarantees.