Imagine you have developed a state-of-art website and after 30 days of worry-free time it finally happens:
Your clients visit the website (let’s imagine this one) and see anything but the expected result: a working web app.
The normal reaction is to call you (in the most unpropriate time) and ask you to fix it asap, because he/she is losing money or the world will end in pain just because this service is not working.
I am sure you will spend 5 min convincing him/her that you haven’t done anything wrong and most probably it’s their fault. Then you will spend another 5 min listening to other-side arguments and after that you will start fixing the problem, without even knowing what it is.