[glyphacki]

I can think of code examples in the documentation. At least you make sure the code is executed or even check the output of the examples.

[DJBunnies]

If only we had a mechanism for demonstrating correct and incorrect ways to use a particular software.

We could expect failure conditions or assert correct outcomes.

It could become a little library of examples that change over time, as the software changes, granting us peace of mind that everything still works.