Recruitment posters most people couldn't read.
ASIO has no trouble attracting applicants. Fewer than 3% get the job. So the posters were written for the 3%: faint copy hidden behind a lined pattern. Seen through a video lens, the lines turn into a moiré and the copy appears. Walk past and it's a pattern. Point a lens at it and it's a message. The other 97% saw a poster.
Try it for yourself below:
A YouTube bumper did the same trick with the player's own interface.