This Star Wars trading card would seem to give us our answer. C-3PO should definitely think about slipping into a pair of slacks, or maybe some cargo pants ... something.
According to StarWars.com, the picture was taken at the exact instant a piece of the robot costume fell off - lining up perfectly and giving the illusion of a fully erect robot penis. Hmmm... I'm not sure I'm buying this explanation. Snopes isn't buying into the explanation either.
This reminds me of the Infamous Sears 1975 Peek-a-Boo Catalog Controversy where a male model on page 602 appears to be sporting more than just underwear, if you know what I mean. Click on the image to enlarge (sorry about the pun). Also, check out Plaid Stallions for more info on it.

And finally, speaking of unintentional exposures, there was the infamous episode 161 of Three's Company where Jack Tripper is wearing a pair of loose fitting shorts and plops down onto a bed, revealing some of John's Ritters, so to speak. The incident yielded one of my favorite quotes of all time. When asked by The New York Observer whether they should edit the scene for future broadcasts, Ritter responded:
"I've requested that Nickelodeon air both versions, edited and unedited, because sometimes you feel like a nut, and sometimes you don't."
