Basically you have a model of your program, but the model breaks down, so you need to check that what you hold true is actually true. Another approach is to us

Basically you have a model of your program, but the model breaks down, so you need to check that what you hold true is actually true. Another approach is to use tools (system stats, logging, debuggers like rr/pernosco, etc). Learn as much as you can about the system and tools you're using. Reduce the size of the problem (reduce code, isolate, etc). Read the code, and try to break it. Build scenarios 'this thing I see in the log can only happen if...' Finally, there's no shame in rubber ducking a problem with a colleague!