"Saff Squeeze"
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.
After banging our heads and tangling up with the debugger, Paul enlightened us about the "Saff Squeeze" technique. Isolating the problem through writing narrow tests where we think the problem/bug might be occurring.
Kent Beck writes about it in "Hit 'em High, Hit 'em Low: Regression Testing and the Saff Squeeze".
To save me from going into more detail, Paul was one step ahead. "Tests aren't just for testing: Stop using the debugger"
