Design by Contract is a fundamental aspect of the Eiffel language.
Methods have pre- and post-conditions; classes have invariants. Those contracts are checked even for heir classes.
This has a very important impact: mocks have contracts too.
Contract checking may be partially or completely removed; this way, Liberty Eiffel is able to generate executables with very high performance.
But when testing, one wants to check all modes. With, and without contracts.
So the tests must be properly equipped. That's what I learned this week: when using mocks in Eiffel, don't forget to bring answers to contract methods.