Everyone makes mistakes, everyone at one time or another has let someone down. But both the wronged and the perpetrator in every instance have an opportunity to take these moments and turn them into something positive.
If you are the wronged party, I would strongly encourage you not to dwell on the negative of the interaction. I would also hope that you would refrain from retelling the mistake over and over again to anyone that will hear, especially if this is someone you consider to be a friend. Go back to the first sentence I wrote–everyone makes mistakes. At some point you will be the person who has wronged another. One of the best habits you can acquire is learning to always speak positively of others who aren’t with you. Sadly, negative gossip travels further and lingers longer. Do you really want to be the reason why others think badly of someone they hardly know?