Yeah, I am guilty of this! I know my boss would shoot me if he knew though!
I work in a shop and there are often times that its dead, so if I am working on my own I'll go out back and look through a few books and take notes for an assignment I am working on and when I see someone walking into the shop through the monitor, I'll just go back out.
But, if you are not working alone ... I guess that could be more tricky!
My boss told me I could bring revision in with me as long as it wouldn't distract me from my work, worked in an off license, but after one shift he said that shelf cleaning should come before my exam! Contradiction much, glad I left!
Not really, but I did take flashcards and kept them in my pocket while I was working at Tesco. If there was a quiet moment or I was taking a break, I could whip them out and refresh my memory. No-one ever bought it up with me, since it didn't stop me from doing my job.