Хорошему вопросу хороший ответ:
Interruptions. Those are the mind killer. Programming requires concentration and juggling facts and threads together as you carefully braid them into logic.
Hot weather sucks. Old computers suck. Buggy compilers suck. Stupid bosses and ignorant clients suck. Demanding schedules suck.
But none of those is a killer like constant interruptions. If you can’t concentrate, nothing will get done even under otherwise ideal situations.