When I took CS50— back when it was C and PHP rather than Python — one of the p-sets entailed making a simple bitmap decoder to get a string somehow or other encoded in the image data. Naturally, the first thing I did was run it through ‘strings’ on the command line. A bunch of garbage as expected… but wait! A url! Load it up… rickrolled. Phenomenal.

Back when I was hiring for a red team the best ad we ever did was steg'ing the application URL in the company's logo in the ad