It is possible to log into the game in offline mode, with "keybounce", when the name registered with Mojang and the login servers is "Keybounce".
The difference – capital vs lower case – does not affect the ability to access your inventory.
However, animals tamed with an offline login of "keybounce" will not respond to you if you are online as "Keybounce".
It also affects access to the player skins.
Tested on 1.4.7.
Your version of Minecraft is no longer supported. We are currently only accepting issues from official release 1.5 and later.