In 1977 the Topps trading card company had the US licence for Star Wars trading cards. It released five series of cards in 1977 alone, including the fourth series against a green border.
Card number 207 in this series featured a remarkable irregularity. As explained on the Star Wars Trading Cards web site:
Card number 207, as it was originally printed, appeared to give C-3P0 an added appendage that would not be particularly necessary for a droid. For this reason, the card is often referred to by collectors as the obscene version of the card. In reality, the added appendage (which might indeed be a trick of the light in the photograph) isn't terribly noticeable at first glance, and far from being obscene, is somewhat humorous in appearance.
Parents of course were not amused, and as rumors about the existence of an 'obscene' Star Wars card circulated, complaints began to come in. Topps responded by reprinting card 207 for subsequent print lots.
It is a myth that the obscene 207 is rare. It is not rare. The same number of 207 cards were printed as any other card in the initial print run. In other words, thousands and thousands of these cards were printed. The same is true of the corrected version of the card. Neither card is particularly hard to locate.
Being Canadian, I never came across this card as a child. The Canadian licensee, Opee Chee, did not reproduce it (or any of the green series). Even if I had I seen this card, as a four year old or even later, I wouldn't have known there was anything obscence about it. But it does make me snicker now. And I'm not buying the "trick of the light" theory.
As Retro Star Wars puts it, "I never knew he had it in him."