Principle of least astonishment
When asked "How do you force a carriage return after each command in a file?", Gary Riley (CLIPS author) replied, "This is one of those cases where the principle of least astonishment applies. All you need to do is put a carriage return after each command."

Perhaps a bit harsh (maybe?), but I thought it was a great "principle". Happens all the time with applications. Someone basically says, "How do I do xxx", and you reply, "You do xxx". :-)