Most security systems' passwords are case-sensitive. Case sensitivity nearly doubles the number of possible characters that can appear in the password, which makes it harder for nefarious users trying to break into the system. As a result, if a user logging into the system has Caps Lock turned on, they'll enter letters in the opposite case and not be able to login. Because the textboxes that collect user input typically are masked to prevent an onlooker from seeing a user's password, the user typing in her password may not realize that Caps Lock is on. To help prevent this type of user error, many login screens for desktop-based applications display some sort of warning if the user's Caps Lock is on when they start typing in their password.
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