I was talking about development and customers today, and remembered an email I once sent to a customer a long time ago.
The customer was complaining that the system that we had developed and that he had been using for a few years did not do a certain thing. We should add it for free because it should have been included from the beginning as it was an obvious requirement.
I used this analogy in my reply:
Imagine you are the Dukes of Hazzard. You have been getting in and out of your car by the windows for years. The doors were welded shut and you did not spend the extra money to strengthen the car while still allowing them to open. However, you always get in and out by the windows so you have never minded this. Now you would like to open the doors and want the mechanic to make this happen for free because obviously doors should open.
He accepted this argument, and gave us the money to fix the doors.