Multiple meaning error message for error code -255

Today, while creating summer school teacher accounts, I came across a user that had been a summer school teacher in the past who's account had not been deleted. When I went to change the password I was faced with the message in the title. Hmmm...that's odd, it's worked for everyone else? Tried a few methods to change the password and still nothing...Till I realized require unique passwords was checked. Once I changed that setting I was able to reset the password and re-enable requiring unique passwords....364 more things to learn this year!