Test the Alternate Paths

Once you have established that the primary functionality works as intended, you can tackle the useful variations of normal behavior. For example, if the primary functionality was to save a file, special accommodations for network file systems might be a good alternate path. At a unit-test level, you might make sure that an event-processing loop functions properly when no events are queued.

At this point, your coverage targets should guide your thoroughness. If you are doing coverage-driven unit testing, then you will want to test exhaustively. You will most likely direct yourself by a sense of functional coverage for full stack system tests. Subsystem integration tests will strive for a more local definition of completeness.