Take a look at this screenshot
You’ll see this dialog when your login credential are incorrect. Now, take a look at this screen
Same login screen, but without the Invalid Username or Password message.
There’s a class of Magento deployment bugs related to sessions and cookies that prevent users from logging into the administrative area. When this happens, your login requests will be rejected but you won’t see the Invalid Username or Password message.
That’s because, behind the scenes, Magento IS logging you in and authenticating your session correctly. The bug crops up because, for some reason, Magento isn’t able to set a session cookie correctly. Common problems include and mismatched cookie domains and site URLs (both on the Apache and Magento System Config level), the server time being set incorrectly (causing the cookie to timeout immediately), or browser based cookie bugs (versions of webkit can’t set cookies correctly with localhost).
So, the next time you’re having trouble remembering that Admin password, be mindful of the error message. If it’s not appearing, chances are your dealing with a cookie problem and not password fatigue.