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If you set it to require both a project and a context, items you put in the Inbox will stay there until you have assigned both. Even once you have assigned both a project and a context, you'll still have to either manually do a clean-up (via cmd-K or the toolbar button) or change to a different view before they will be swept off to their ultimate destination.

If you set the preference as suggested, you can move items back to the Inbox from your Misc container by selecting the actions, bringing up the inspector with cmd-shift-I (or clicking the toolbar button), and setting the Project back to Inbox (should be at the top of the list).