When a player who has never joined the server before joins the game, their info cannot be loaded in the control panel. Requesting to do so will yield a loading page that will never load.
This effect disappears as soon as the player logs out, as predicted by @Anna
Once the affected player logs out, their data can instantly be accessed via the control panel and the issue is never to be seen again.
This issue can be seen by visiting http://minecraftonline.com/cgi-bin/controlpanel?playerinfo=x where 'x' is a player online that have just joined, and has not left the game before.
Tested on player lemons222. Due to the nature of the bug, replicating it may be an issue without either deleting existing player data or using a fresh account to login (I have an alt that I can login with to help test if necessary).