Casual APIs

This is basically the second half of the Robustness Principle[1]:

  Be conservative in what you do, be liberal in what you accept from others.

If you use both halves things get even better. For example this is why digital electronics work — circuits are forgiving about the voltage ranges of their inputs but very precise about their outputs.

It's also a nice way to live life :'(

[1] http://en.wikipedia.org/wiki/Robustness_principle