I came across this bug when creating an account for the Public Mobile Community forums.
There are no character limits for passwords when you create an account but passwords are limited to 20 characters on the sign in page.
This means that you can create a 20+ character password when you create an account but you won't be able to sign into the account once your session expires or if you sign out.
You can reset your password fairly easily but it would make more sense to impose a character limitation for passwords in the account creation screen.
