One main goals of every program is reliability, that is, correctness and robustness. A program is correct if it performs according to its specification, it is robust if it handles situatios that were not covered in the specification in a graceful manner. One way to prove the correctness of a program with respect to a (formal) specification is the Hoare calculus. Based on this formal method Bertrand Meyer developed a method of software engeneering called Design by Contract.
The principal idea of Design by Contract (DBC) is that a class and its clients have a contract with each other: The client must guarantee certain conditions before calling a method specialized on the class (the preconditions), the class guarantees certain properties after the call (the postconditions). If the pre- and postconditions are included in a form that the compiler can check, then any violation of the contract between caller and class can be detected immedeately.
The language that offers the best support for DBC is Eiffel, designed by Bertrand Meyer. It is rather difficult to add support for DBC to most other languages, but not so for Common Lisp: I have written a package for Common Lisp that provides support for DBC. It is still very new and not too well tested so you should expect some rough edges and changes in its future design. There is no larger program depending on this package available, only some silly test cases. Since I intend to use the dbc package for my own programs this should change in the not so distant future.