I have my 2011 iMac permanently plugged into my TV via HDMI. For most of the time, I just keep a single display (the iMac), but I also watch some programs on my TV, so switch the display to duplicate on the TV and the iMac when doing this. When I've finished using the TV, I switch the display back to iMac only.

Often, once I've switched back to a single display, many of my windows are not visible. For example if I click the Finder icon, it appears as though nothing is happening. The same happens for other applications. The applications are still open, but are being displayed out of the range of the iMac screen. If I switch back to mirrored display (iMac and TV), then I can see the open applications.

To fix the problem I need to drag the applications back to the iMac screen while on mirrored display, then switch back to single display (iMac).

In my opinion, when switching back to a single display (iMac), then all of my open windows should be visible on that display!

Has anyone else had this problem?

Also, this used to happen under Lion, but it MUCH worse under Mountain Lion.