I recently did a clean Windows 7 install and reinstalled DF 4.01. I normally run Mumble (a VOIP app) on my second monitor and now whenever I close Mumble on my second monitor it remains in the DF taskbar on that monitor. If I move Mumble to my primary monitor the app moves to the Windows taskbar on that monitor and closes properly. Other apps I run on my second monitor don't behave this way.

UPDATE

As I was writing this post I wondered if Mumble was somehow still running after closing it so I ran Mumble on my second monitor, closed it and then opened Task Manager and Mumble was in fact still running- killing the process in Task Manager removed the app from the DF taskbar so now I'm thinkin' I have a Mumble problem.

UPDATE 2

Uninstalling and reinstalling Mumble fixed my problem