Editor can't open dialogs or menus, running out of USER handles

Unity 4.2.1f4 (Windows) appears to have a USER handle leak.

If I leave the editor open for several days, hibernating my machine at night, after a few days the editor will be unresponsive. You'll click on a menu and nothing will happen. The menu bar depresses, but the menu does not appear. Closing the window when a save is required and nothing will happen, the Save dialog never appears and the app never closes.

I opened Process Explorer and looked at the process and I see the following:

Notice that the USER Handles count has hit the Windows maximum.

It's possible one of my many shell extensions (like Dropbox, WebRoot Antivirus, Start8, DisplayFusion could be causing it), but no other app suffers from the issue.

Anyone else seen anything like this?