In 1978, there was a pornographic Star Wars trading card issued by the Topps company in the USA. It was card #207 in the fourth series (aka the "Green Series", so named for the color of its border).
It contained a picture a C-3P0 with a certain metallic appendage. No one is quite sure how the boner occurred, but myth states that it was a disgruntled airbrush artist who added the appendage to the hapless 'droid.
The additional artwork was eventually noticed by Topps personnel, but not until thousands had been collected by unwary 10-year-olds all over North America.
According to the Star Wars website, the idea of a disgruntled airbrush artist has been discounted. After reviewing the archives at Topps and Lucasfilm, they figure that it was all a matter of coincidental timing.
The theory goes on to say that the photo was shot at the same instant that a piece of the C-3PO costume started to fall off. It lined up at such an angle to suggest that C-3PO was not only a protocol droid but a galactic love machine.
This was determined by reviewing the original untouched contact sheets which showed the same image.
Instead of trying to explain to horrified parents that it was all due to bad timing, Topps simply reissued a retouched photo which airbrushed C-3PO's extra package to oblivion. This corrected version was actually scarcer than the original as its distribution run was limited. However, the original is still worth quite a bit of money to collectors.
- Snopes - http://www.snopes.com/business/hidden/c3po.htm#add
- Official Star Wars website - http://www.starwars.com/episode-iv/feature/20000530/