An oldie but a goodie.
When you write / design / create, be it a building, a program, a document or something else, try and think what reaction would cause the least amount of surprise to some input (eg. if you walked down a hallway and came to a dead-end, you would probably be surprised, ergo don't design dead-ends into hallways for no reason).
Why then, does the Windows XP "ctrl + alt + del, u" always shutdown straight away, unless there happen to be 50 updates waiting to be installed? Now I'm stuck waiting for half an hour with a dead computer, because the shortcut I always use to shut down has changed from "shut down now" to "install updates and then shut down".