When creating a standard username/password login page, care should be taken to ensure that malicious users are not given clues about the nature of the login system.  The following error messages should be avoided when displaying a failed login attempt.

"The password for this user is incorrect" - This confirms that the malicious user has a valid username.

"The password should be 6 characters long" - The malicious user now knows the length of a valid password.

"The username could not be found" - The malicious user can simply keep trying until they enter a valid username.

Displaying a more generic error message along the lines of "The username or password you entered is incorrect" offers fewer clues about the nature of the login system in use.  This makes it just a little bit more difficult for someone to login using someone else's account credentials.