In addition, users often use the same (or very similar) username/password pairs for multiple systems. This is convenient, but can be dangerous in some circumstances when the systems involved have very different levels of security or for whatever reason the administrators of one should not be able to gain access to the other. For example, it would be dangerous to re-use a work password to sign up for a contest on an untrusted site. The work password may be able to access sensitive information such as trade secrets, customer databases, or payroll information. Especially with web logins, users are often asked to provide an email address and a password, and the email address and other information requested could be used to figure out the system and username on which a password may also be used. Yet, if you want to be sure of remembering the ``new" contest password, reusing a password would be one way to ensure that you do.
Sometimes, because of varying rules (see Section 2.2), users will be forced to choose slightly different usernames and passwords even if they intend to use identical ones. This then increases the likelihood that a user will mis-remember or confuse passwords when attempting to log in.