I was looking at the images found in aero.msstyles
when I found that the checkbox has a (supposedly unused) variant of the checkbox with a cross instead of a checkmark. I quickly confirmed that this had been the case since Windows 7 (or Vista).
Here is an example of what I'm talking about:
Now, what's the purpose of this variant? Is it unused and just included for completion sake, or is it something that can be seen in the wild?