There's probably a very good reason for this, but anyway: I had to have a locksmith come out last week, and completely re-key all the doors (employee was escorted out of the office - security procedure).
I had 2 staff members who were on vacation last week, and I had left them messages that their keys didn't work, and for them to not try to come in early, because they would not get in if someone else wasn't there.
Monday morning, both employees were able to unlock the new locks with the old keys. I called the locksmith, and he told me that was impossible, but he would come out. Well, he couldn't explain but we were right. He even showed me how all the old keys were identical, lined up exactly, but two of them would unlock the new lock, and all the others wouldn't.
So if he couldn't figure it out, I don't feel stupid. He also redid everything for free, and the old keys definitely do not work.