When you go to the realms and then click "Buy a realm" while having no internet connection the "Back" button won't appear and you can only quit the window by pressing ESC.
Also you get only the first time the Error 500 (no connection) and end at the start menu. Every time you try it after this you will just get this:
[13:22:28] [Realms-stat-message/ERROR]: Could not get stat
but it won't cause a error message in your client.