Mission Control Screens Rearrange themselves

Let's suppose I use Desktop 1 for Mail and for Safari browsing on my iMac.

I then use Desktop 2 where I run Parallels for my work related programs that require Windoze 7 applications.

For some strange reason, Desktop 3 routinely gets shifted into position 2 and I notice a different desktop background that was originally assigned to Desktop 3. Desktop 2's background gets pushed to position 3...

Anyone else have weird stuff like this going on?