A computer always stores a user's username and password.

The assertion that a computer always stores a user's username and password is false because not all systems have the same approach to data storage, particularly concerning sensitive information like usernames and passwords.

In modern operating systems and applications, security best practices dictate that passwords should not be stored in plain text. Instead, they are typically hashed or encrypted using secure algorithms. This means that the actual password is transformed into a different string of characters, making it very difficult to retrieve the original password from this stored data. Usernames, while less sensitive, are usually stored alongside password hashes but do not equate to the ongoing storage of actual credentials.

Additionally, many systems utilize temporary sessions during which usernames and passwords may only be stored in volatile memory (such as RAM) and not written to disk. This minimizes the risk of exposure if a device is compromised. Furthermore, users often have the option to not save their credentials, which also contributes to the falsity of the statement.

Hence, the statement aligns with the understanding that while there are mechanisms to manage user credentials, such as session cookies or password managers, it is not accurate to assert that a computer always saves a user's credentials permanently.