'The street has its own uses for things', wrote William Gibson; Field Marshal Helmuth von Moltke also wrote that 'no plan survives contact with the enemy', or in this case, the players. When developers introduce a feature or content of some form or another, there's always the possibility that players will use it for a completely different function.
Take City of Heroes, for example, where the ability of some desks in the base designer to have objects stacked on top of them has led to a whole school of base design involving levitating objects. Also in CoX was the superbly inventive use of a dropped oil slick as the target for a mass rez power, causing one of the developers to exclaim 'wth?!' when he saw what the players were up to. And that's not even touching upon the use of pink-tinted gas masks as surprisingly effective pig snouts in the costume generator. (Don't ask.)
What repurposed content have you seen in games? Have you come across (or instigated) any new uses for content that were better than the original envisioned design?