How recently were they closed? If you close a window and instantly press Alt+Tab, this can happen, because DF hasn't had a chance to remove the window from the list. The Alt+Tab polling for updates is done in the background, so that the Alt+Tab window will open as quickly as possible, and so that the contents don't get shuffled around after it's been opened. An unfortunate side effect of this is that windows that were closed within the last 1-2 seconds may show up still

Sounds good! You can just reply to this thread if it starts happening again

Thanks!