I have an old Schlage SecureKey installation. (Yes, I know about Schlage having lost the lawsuit almost a decade ago.) I haven't had any problems with the lock.
Recently, a weird thing happened. Leaving the house, and locking the door, the lock felt strange, like there was some resistance in it. Moments later, I turned around to re-enter to fetch a forgotten item, and, to my surprise, the key would not turn: I was locked out!
I experimented with the lock a little bit, and discovered that that it had got into a strange state, as if the combination had jumped one pin over. So that is to say, I was able to open the door by inserting the key only one position short: four pins, instead of all five. To my surprise, it turned! I tried all copies of the key, they all now worked like this.
Now I happen to have two reset keys for the lock: one for the current key (let's call that A) and one for another key shape (let's call it B).
I tried doing a "null reset" on the lock: A to A: put in the A reset key, rotated it to the programming position, pulled it out, put it back in, rotated back, withdrew. Firstly, I was surprised that the reset key worked, having been inserted all the way, like it is supposed to be even though the regular keys which have the A shape worked only in the partially inserted position. Anyway, A to A "null reset" that didn't accomplish anything, and may have made the problem worse. I seem to recall I was not able to turn an A key at all after that. Then I tried doing an A to B reset. I confirmed that the lock took the B keys just fine. From there, I programmed back to A. Now everything seems good! The A keys all work fine again.
Can someone explain ... what the heck? And should that lock be replaced?
Oh, and one more "key" piece of information: recently I filed down the teeth a little bit on the key that I use because I got annoyed with it being a hard to withdraw: not from that lock, but from another ordinary lock keyed for the same key. I didn't mess with the notches of the key, of course, just the teeth/ridges, so the key works fine. It seems like too much of a coincidence: this deprogramming incident happened within two days of introducing the filed-down key, after "years of trouble-free operation".