I'm not sure if this is the same thing, but there's a "feature" where if you hover over an open application on the taskbar, it will not only show its own open windows, but also a weird "combined" view of that application and whatever else happens to be nearby. So you might have both Notepad and Outlook open. Hover over Notepad, and you don't just see a preview of the open Notepad window, you also see this stupid "Notepad + Outlook" window.

It's completely useless and so frustrating. Sadly it's impossible to get rid of.