This happens to me
I play window mode, so if you do too I suggest that azurerogue's conclusion is correct. I don't know if there is a fix-it for this, but you could work around it by hitting print-screen, then going to ms-paint, and do a ctrl-v (paste) and save into the screenshots folder
#2 This method lets you easily discern the screenshots you want to save from the screenshots you take by accidentes
#3 When you want to clear your screenshot folder cause its oh-so-full-of-little-accidentes, just nuke all the grey skewed ones.
toodle-ooo~