Your reply got me thinking too. My immediate knee-jerk reaction was to write something along the lines of intentionally vs. not, but then I stopped and thought about it some more and...

Imagine your good friend promised you with the house move. You agree on the date and... he does not turn up. After you've done all the job yourself, you call him and ask, what happened? "Sorry, I mixed up the dates." OK, never mind, you move on.

Months later, your car breaks down. You arrange a fix at the workshop and your good friend promises you to tow you there. But again, he does not turn up on the day. "Sorry mate, I was sick."

A few more things like that happen and the next time you need a help with something, you no longer ask your friend. Not because you think he would screw you up maliciously. Simply because you cannot rely on him. Does he stop being your friend? Not necessarily. But you no longer think of him as someone you can rely on in time of need.