To repeat an old post:
There’s a cool little exhibit at the Australian War Memorial, in the Boer War section.
It’s an image of a rocky hillside. The sign beside it explains that the Boers were one of the first to employ modern-style snipers, and asks “can you spot the snipers in this picture?”.
They’re very hard to spot, so people tend to lean in close to the picture to examine it. And they’ve got it rigged with a motion detector that sets off a loud gunshot sound if you lean too close.