This has been a very sporadic issue that I've seen once or twice per year at most, for quite a while with OS X - somehow, another window is able to steal focus

This has been a very sporadic issue that I've seen once or twice per year at most, for quite a while with OS X - somehow, another window is able to steal focus from the login screen. I've never been able to reproduce it reliably or find a common element in all of the times it has happened, but it definitely has happened to me and I've also seen co-workers dropping their login password in a chat window due to this. But it is pretty rare, so hard to pin down. I've also noticed another thing happening more lately - locking the screen, only to have it automatically unlock itself a second or two later. I always have to make sure it actually stays on the screensaver for a few seconds before I trust it will actually lock.