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You're making the same assumptions you made when you wrote the module you're testing.
One way around this is to do what large corporations do - have someone else write the tests. The developer provides the tester with the design specifications and nothing else. In fact, it's better if the developer and tester never speak of the specifications at all, to avoid contamination. Then, the tester writes tests to exercise the specification, not the component.
That last is so important, I'll say it again -
Remember - the specification is everything. The component implements it and the tester tests against it. It is your whole world.
Component testing has another name - unit-testing. Testing the implementation against the specification is something that the developer cannot do.
Now, this doesn't mean you have to have an entire testing team. At one position, we did cross-testing. I develop something, then hand another developer the specification I developed against. Based solely on that, the other deveveloper tests my implementation. You'd be surprised how many bugs are found with just this simple solution. Only when the cross-tester ok's the implementation is the test team even notified that the specification has been implemented and is ready for their testing.
Automated testing, imho, should be built from the test-team's tests. When I worked as a test-tool supporter for Motorola, the developers did unit- and cross-testing. (They may have also done some regression testing, but I wasn't aware of that.) It was then handed to the test team. The test team did both regression and integration testing. Once that was completed, a subset of the integratration tests was chosen and added to the regression test suite. We also had a regression test team that was involved in doing the complete regression suite. (When I was there over two years ago, the regression suite for any version of Motorola's BTS software exceeded 10k tests. A full automated run took almost 80 hours. I'm sure it's larger now.)
The idea is a little like C++ templates, except not quite so brain-meltingly complicated. -- TheDamian, Exegesis 6
Please remember that I'm crufty and crochety. All opinions are purely mine and all code is untested, unless otherwise specified.