We live in paranoid times in which people are apt to interpret harmless objects (such as battery-lit sweatshirts or cartoon characters) as imminent threats. But we're really no more paranoid than previous generations. A case in point is the Nazi Air Marker Hoax of 1942.

On August 10, 1942 the Army public-relations office issued a press release warning the public of "secret markers" that had been found on farm fields throughout the eastern United States. These markers were patterns formed by the arrangement of fertilizer sacks or the way a field had been tilled. From the ground they looked like nothing, but from the air they formed the shape of arrows, apparently created by Nazi sympathizers in order to guide enemy bombers straight toward military factories and airfields.

There was a big public outcry. Editorials warned of the need to guard against the enemy within. But a few days later it turned out that the "secret markers" were really just random patterns. A case of military pareidolia. The Army admitted the story "may be untrue." In one case the pattern had been created by the Department of Agriculture, which had directed a farmer to plow his field in that way to help stop soil erosion.

More about this (as well as pictures of the "air markers") in the Hoaxipedia.