Illustrator CS6 (win7) eyedropper is misbehaving

Question

The eyedropper tool in Illustrator CS6 is causing me great frustration and i'm seriously convinced that it's buggy.
Ordinarily it will select the "Appearance" of a vector, but sometimes (compleatly randomly as far as i can tell) it will start to only sample the direct colour of the pixel under it, this includes the RGB colours of guides, edge-outlines and even the lime-green smart-guide annotations.
What's worse is this strange little bug/setting seems to stay with a file once saved. I've doubble-clicked the eyedropper tool to check what options are selected (all except the top-level appearance checkbox) and they are also the same in a file not displaying this oddity.
I hope I've justed missed some unknown keyboard short-cut or some strange setting.
[We're running CS6 on six Windows7 machines and have experienced this issue on all of them]

Answer 1

Thanks for the reply, you're correct in that it's the exact same function that the shift-key ordinarily performs.
But I doubt it's a driver, it's on six different computers and only on files that have been saved with the issue.