Under certain circumstances, you can instantly cancel your connection to a server if you double-click to join.
- Connect to a server by double-clicking on its name in the server list.
- The position of your mouse pointer when double-clicking the server name must match the position of the "cancel" button which appears while connecting. (This normally happens if you click the 6th or 7th server in the list, depending on the size of your client window).
- The "cancel" button activates even though it has not been clicked, and you are returned to the main game window.
(I believe this is the same bug
MC-2001 was trying to describe).