[kevan]

>Somewhere your model of the code is broken, and you need to test find the breakage (model needs correction, or program needs correction).

Most of the harder bugs I run into fall into this category. Great list of techniques to use. For when I'm really stumped I go into a more formal scientific method to the splitting the problem space approach. Formulate a hypothesis that if answered will reduce the search space, either by ruling in or ruling out things. Write it down - this is the important part, test it, repeat.

It's slow but usually helps me keep forward momentum. Worst case it at least creates a list of things that need better logging to figure out what's going on