A couple of misapplications of physics come to mind:

Conservation of angular momentum does not mean what people think it means. If you have an object spinning on a flat surface, it can't turn around without outside forces, right? Wrong, the [rattleback][1] toy does this ([video][2]). 

The [Coriolis effect][3] is real, but the idea that this has something to do with the direction water spins down the drain is a [false][4] [urban legend][5].


  [1]: http://en.wikipedia.org/wiki/Rattleback
  [2]: https://www.youtube.com/watch?v=LmEf7aIhpF8
  [3]: http://en.wikipedia.org/wiki/Coriolis_effect
  [4]: http://www.ems.psu.edu/~fraser/Bad/BadCoriolis.html
  [5]: http://www.snopes.com/science/coriolis.asp