That's just common (engineering) sense. In his beginner lesson on programming a Poker game with Python, Peter Norvig ask the student to write a test for a 1000

That's just common (engineering) sense. In his beginner lesson on programming a Poker game with Python, Peter Norvig ask the student to write a test for a 10000 cards deal. That's the same. "It just work" is the user's resulting feeling for a product that is working well under any conditions, known and unkown, aka a well-engineered product. (Which often means getting over the project manager's head) This means to have few to no implicit assumptions in the code. Vim is a great example: developped when a 10kb file was a huge file, it will kindly open and edit a 2Gb log file for you (if you don't have all these silly 3rd party plugins). Vim may be burping a bit on very very long lines, but it still shows that the running code behind is not assuming on your behalf that you are editing a couple of haikus.