I suspect a lot of the time this comes from the fact that it won't let you use any previously used password.

So it goes like this:

1. create account, set password one
2. forget password one when trying to log in, quickly reset the pass to get in, setting a new pass in the process
3. forget that you reset the password, but remember password one again, so you try to use it
4. doesn't work, so you reset the password again, trying to set it to what you thought it was (password one)
5. get this error, because while password one is not the current password, it has been used before

It's still funny, because it happened to literally everyone, humans just work that way.