Most programmers above a certain level are very good at conceptualising large, complex systems. They can interrogate perceived weaknesses in a program before it is even written. This is how good programmers manage to write programs that are largely free of defects and that generally work in the way that is expected. Small-scale errors aside, a good high-level conceptual understanding of the system, coupled with an ability to mentally simulate the behaviour of any portion of a program provides programmers with most of the tools they need to construct good-quality programs. The programmer is likely to know (at least at some high level) how the entire system is going to work any any given moment.
From Adventures in Programming Languages and Semantics, "If concurrency is easy, you're either absurdly smart or you're kidding yourself." The full article is a good read.