A life lesson-save the excuses.
Excuses might make you feel better, but really, they don't make anyone else feel better. Excuses can sometimes serve the purpose of explanations, but really, that only works if the person on the receiving end is looking for an explanation.
Shit happens. People get sick, hurt. The car breaks down. The person who was supposed to be there for you fell through. But the bottom line is something didn't get done, that you were supposed to do, or something got done that wasn't supposed to, and it's your fault. No point in defending yourself; that only makes the party you're making excuses to more annoyed.
So what do you do? Try this simple phrase, "I'm sorry I disappointed you. I understand how you are upset? What can I do to make it better? Or, "I'll be more aware of that in the future".
Owning up to your failings, even when they are beyond your control shows the other person that you are accountable for yourself, that you own your failings, and that you want to work with them. Excuses show weakness, lack of accountability, and an incessant need for self-preservation.
So, try it sometime. Don't make excuses. Stand up and show someone that you are a dependable person, even when you fall short of expectations.