Now, interpolated grid lines curving beautifully in two directions appeared on my screen. The lesson here is that testing must be done on many kinds of datasets, not all of which obscure possible errors. Generating good test data is a problem in itself, and mine had all been nearly rectangular enough to have largely hidden a definite mistake. Seeing nice looking plots of results is one of the rewards of this sort of work, and they had seemed fine back then. Now, seeing them look fine for this awkwardly shaped dataset is even more rewarding.
Computers have a way of ruling our lives.
This was just one more instance of a hazard in the life of a software engineer.