I knew a guy who worked at one of the national labs that had its own Cray supercomputer, in a computer room with a big observation window that visitors could admire it through, of course (all Crays required observations windows to show them off).

Just before a tour group came by, he hid inside the Cray, and waited for them to arrive. Then he casually strolled out from the back of the Cray, pulling up the zipper of his jeans, with a sheepish relieved expression on his face, looked up and saw the tour group, acted startled, then scurried away.