There are so many things that could go wrong in delivering a piece of code. Just the ones I've seen enough times to build rules of thumb about exceeds my own powers of recall. And some people reliably trip over certain failure modes, while others avoid them subconsciously, never even realizing there was a hazard to begin with.

My new motto is: filter out the reversible decisions and worry about the rest. When something in the former set breaks, make it a teachable moment.