In my experience, places that offer a warranty on a "guaranteed" lifespan tend to have lower quality stock, so they do tend to die inexplicably, more often. My LFS refuses such guarantees, and yes, some of the fish I've bought from them have died, but the shortest lived ones were full grown when I bought, and lived for over a year. From the big box stores that offer the guarantees, at least 1 out of every batch has died on me for no good reason.