Skype may prevent Outlook from closing properly

If you use Skype with the View / View Outlook Contacts option checked, Outlook does not close properly after you hit Alt+F4 or click the Close button. It disappears from the taskbar, but the OUTLOOK.EXE process continues to run and the Outlook icon stays in the system tray.

I think this is because Skype holds some kind of lock on Outlook's address book, but that's only my guess.

Here you can find some other applications that cause similar behaviour.