This happens to me on average around once a week, and happens to a number of
our developers about as infrequently.

If you lock the screen on your machine, when you come back the password
dialogue is showing with a message saying "Authentication Error" or
"Authentication Failed" underneath it, you can see the password dialogue is
flashing slightly as if a key is being pressed.

If you try to type your password, you cannot completely type it in as every
time you attempt to type it the characters disappear as you type. However, if
you go to a tty, there is no such issue with typing, you can log in as normal.

Generally the only thing I can do at this point is reboot and hope I don't lose
too much work.

In my setup I have a wireless keyboard plugged in, but even if I remove it, it
doesn't stop. I also tried pressing every key on the physical keyboard to
ensure the keys were not stuck, and they all seem fine.