Lady spinning on escalator handrails. Nominally the net force is zero, assuming each rail exerts the same magnitude of force, but there may be some differences depending on what part of her body is in contact, or the escalator speeds. They each exert a torque in the same direction, though, so she spins.
via Kottke
Of course, the two moving rails aren’t acting along the same line, so even if they produce the same force, they make a couple, which results in her spinning.
http://www.break.com/index/escalator-spin-fail.html