I had a normal locksmith call this morning to unlock a house. The lock was a grade 3 Schlage leverset. I put my tension wrench in the keyhole and as I put tension on it, the lock core started to turn and unlocked the lock. I hadn't ran my rake through the lock prior to this. Once I go t inside, I asked the owner for their working key. I tried it and it worked. The only thing I could figure at the time was that there were no tumblers in the lock but as I looked into the keyhole I could see a tumbler in chamber number one. Just out of curiosity, I tried a non-working key. When I turned it, the lock didn't turn. Continuing on, I inserted a flat screwdriver in the keyhole about 1/16" of an inch and was able to turn the lock and lock and unlock the door.
In 20 plus years, I have never seen this. I didn't have time to take the lock apart but am still curious as to why this happened.