Many years of painful lessons, and finally I realized that whenever I "believed I could keep all of the state and context of [whatever] in my head", that belief is usually unfounded. Eventually being bitten by this (and chided by GTD people) enough times convinced me that your brain should be stateless - all state and context should be stored somewhere. Even if you're capable of keeping it in your head, it's a suboptimal use of brainpower that could be better used in other ways.