In 1977, Topps issued a set of Star Wars trading cards. The set included one card that would, arguably, become the most infamous card it ever printed. This was the so-called C-3PO 'golden rod' card — so called because it seems to show C-3PO in a state of prominent arousal.
Mel magazine recently posted an article
which explores the history of this card. The article makes a couple of interesting points:
First, Topps not only really did issue this card... it printed a lot of them. This isn't a rare card.
Second (and to me this is the most remarkable thing), no one involved in the creation of the card noticed its most salient feature until after it had been released and word started to spread among card collectors.
Finally, there's no evidence that the card was the work of a rogue artist. The most compelling theory about what happened is that a panel on the side of C-3PO's armor plating must have accidentally fallen open, and was in exactly the right position to create the illusion of robotic arousal. And then, for whatever reason, Topps selected that image, out of all the possible ones, to print.
Eventually Topps released a corrected version of the card (below). Apparently this corrected version is rarer, and more collectible, than the original. Though, ideally, a collector would want to have both.