I've bought quite a few Steam-redeemable bundles lately, so I've been installing a heap of games on Steam. I suddenly noticed that on my Steam library "The Binding of Isaac" was marked as uninstalled, which I knew for a fact to be wrong.
After taking care to preserve my save files, I went ahead and told Steam to install the game, which apparently jogged its memory (but only after "preparing" the game's files for installation).
So while there appears to be no harm done, I've never had this happen before, and so I wonder if this is some sort of ominous sign.
Best Answer
I imagine that the library cache file became broken you could probably rebuild it to get it to remember but steam should check the location before installing and see what is there and since your games will be all installed then it should not hurt anything. the real issue is why is the library file/s getting corrupted?
But in all you should be fine and there is nothing to worry about. It has happened on a few of my games before and all has been well.