Here is a pseudo rule for debugging: Don't make guesses. What you should do instead is to compare runs that exhibit the problem with those that do not. That way you can narrow down the exact effects which contribute to the problem. This leads us to the Divide and Conquer rule I wrote about previously.
Theories about what the problem might be are not worth much. In fact, the can be counter productive. You will most likely waste time tyring to check whether your theory is correct or not. You end up right where you started, except it is much later in the day.
Now this is not to say that you should never harbor a guess at the source of a bug. You should just limit the amount of guesses you make. Once you get good at debugging, you can use few guesses to get to the source of the problem.
Stay tuned for a commentary on the use of tools for debugging. I will leave you with another debugging gem. Change but one variable each time you run a test. This will help focus your cause and effect research.
Mysterious Double Instance Hampering Performance - I study the existing code base. Confer with a colleague. Then I determine the optimal plan to change the functionality to load only a slice of all the dat...