Of the five metro routes I mainly use in this town, I only find the unintended footrest in the type of bus that regularly services the 36 route on Beacon Hill. The footrest is located behind the long window-facing seat at the back of the bus. One must sit at the very corner to enjoy this resting place. If you are on the left corner, your left leg is afforded the pleasure, and on the right corner, your right leg. (There is, if you didn't catch, a little dub in my use of the word "afford" in that sentence.) The way the black paint has been worn off indicates the extent to which this fixture (whose primary function is securing the seat) has been repurposed...

Exaptation and the related term co-option describe a shift in the function of a trait during evolution. For example, a trait can evolve because it served one particular function, but subsequently it may come to serve another. Exaptations are common in both anatomy and behaviour. Bird feathers are a classic example: initially they may have evolved for temperature regulation, but later were adapted for flight.