A true friend is someone who will not hate you, or otherwise dislike you, for times when you accidentally screw up on something.
Let's say you and your friend had planned to meet up somewhere at a certain time. Your friend shows up, but you couldn't, and the reason you couldn't was because maybe your car broke down, or you weren't feeling all that well, or you had to take care of an emergency.
In a situation like that, a true friend would understand the explanation you provide him and not hold it against you for not meeting up with him. If you see that he's acting all pissy and bugs you for "ditching" him, then he's not a true friend and you should block all contact with him.