Alt+Tab issue during verfication

I have a strange issue that I just recently figured out. There are times when I use Alt+Tab to switch windows, the alt+tab box with all of the windows on it stays on the screen. I finally figured out that it only happens during file verification.

If I right click on the icon in the taskbar and do a cancel, the Alt+Tab window goes away. Then I press No I don't want to cancel. It happens every time I go to switch windows (it's a habit I have) only during verification.

Any idea why?

Thanks.
Jan