My secret weapon for bug diagnosis is that when a regression is reported on a system without an automatic bisect tool, while everyone else is trying to reason about the problem with guesswork and code inspection, I sit down and spend 2 hours just bisecting manually (full sync, rebuild and install of old versions of the software). This provides a guaranteed culprit CL, often one that no one guessed, and also a potenti…
My own personal experience with difficult bugs is that there is no substitute for taking the time to understand the problem domain, the systems involved, and the code itself. Getting to that point takes significant investment though, and I tend to trust engineers who are willing to do that much more than engineers who don't.