By all means, tests should evolve while the program is written -- however boundary coverage should be one of the first kinds of things you think about when testing your API. That is what automated testing is for. If your API passes all tests, and the whole of your API is tested, you won't have any issues with coverage...harder said than done.
See the reply to adrianh above regarding internal/algorithmic boundary conditions.

TDD is an *excellent* practice. Didn't I start out with that line? I am only trying to get the point across that it isn't the be-all and end-all of testing, it has limitations.

Steve McConnell's Code Complete 2nd Ed. cites research indicating that while developers think they are getting high code coverage in testing, in reality they get 80% at best, 30% at worst, and around 60% on average. 60% is very, very, very low code coverage. The studies aren't recent, but if anything has changed I suspect it is only that developer confidence has risen, not that test coverage has.

In reply to Re: Re: Re: Re: Automatic generation of tests by Anonymous Monk
in thread Automatic generation of tests by DrHyde

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