In Hit ’em High, Hit ’em Low: Regression Testing and the Saff Squeeze Kent Beck outlines a very nice technique for isolating a defect using regression tests:

“To effectively isolate a defect, start with a system-level test and progressively inline and prune until you have the smallest possible test that demonstrates the defect.”

Kent provides a step-by-step example of the technique, which he attributes to David Saff. I’ve always done the “High” part of the process, but the successive inlining and pruning to get to the “Low” is new and very clever. Tucked into my bag of tricks now.

(Link via PierG.)