There are more important things to do than to carefully comb over or instrument thousands upon thousands of lines of other people's work (if that's even an option; it's very often impossible eg closed libraries) for the sake avoiding one of the most useful tools a developer has. When you're using a debugger, the number of assumptions that you have to make about what's happening drops to almost zero. If you are making no assumptions without a debugger or ridiculous levels of instrumentation, the software you are working on is of trivial size, full stop.