Some markets, especially the small ones, go out of business, fold up their tents and disappear overnight. Since they usually don't pay until publication, you would be hard pressed to call this a sale, although you might have considered it one before learning of the market's demise. Sometimes you don't, and a letter saying they want to buy your story is not a contract. Well, even when you do get a contract, often it's still not a sale, by some way of thinking. A contract can be worthless.
Many contracts are all in the publisher's favor and offer the writer no protection whatsoever.
In contracts like this, the writer assigns First Serial Rights to the publisher, who agrees to pay some amount when the story is published. The writer has nothing, and is contractually prohibited from marketing the work elsewhere. I don't know that any markets intentionally do this, but they could. It costs the publisher nothing and could hold up your story forever. To protect myself against losing a story to literary limbo, I have always added a two-year reversion clause to every magazine contract that didn't specify one, and I have never received any objections.