If a user can move to a part of the page via the keyboard, the user must also be able to move away from it using only the keyboard, and if it requires some combination other than the normal exit methods, the user must be told how to move the focus away before they enter that area.
No Keyboard Trap: If keyboard focus can be moved to a component of the page using a keyboard interface, then focus can be moved away from that component using only a keyboard interface, and, if it requires more than unmodified arrow or tab keys or other standard exit methods, the user is advised of the method for moving focus away.
Note: Since any content that does not meet this success criterion can interfere with a user's ability to use the whole page, all content on the Web page (whether it is used to meet other success criteria or not) must meet this success criterion
How to fix:
This is often an issue on pages that rely on plugins, such as Flash or Java applets -- the user can tab in, but the plugin has no provision to tab back out. Make sure that the plugin returns the keyboard focus back to the parent page after tabbing through the plug in's tab sequence, or make sure there are instructions on the page, accessible before the plug in is entered, for exiting the plug in context.