You know that friend? The one you support through thick and through thin and you always always always will have their back? The friend you think the world of?
That friend you think the world of until the world seems so important? The one you somehow miss beyond your own shadow? The friend you suddenly have the nerve to doubt despite all you have been through?
That is a good friend. Don't be like that.
What a lot of people refuse to acknowledge is something that should be so incredibly obvious: Words mean things. The things you say--each word means something quite literally, even if you don't think you intend them that way. It's why I don't believe in frivolous cursing and obscenities. What you say or write with people around will be captured in some way. And I know for a fact you will regret it later.
The truth always comes out. And as a person whose highest value is in what is true, I can really respect that.
The problem is that we aren't always true to ourselves and go along with our 'selective hearing'. We'd rather be 'correct' than right, and the pursuit of that always ends in disaster. Maybe it won't for you upfront, but know that that tiny piece of your conscience will become a bigger and bigger chunk as you become more comfortable with this moral compromise. Don't let that happen. Take a look in the mirror.

One person I know deserves this apology from me. (They know who they are.) I am deeply sorry for the mistake I made, and I hope they can still forgive me though the things I have said away from them have undoubtedly hurt. That is not the person I am. I am better than that, as they have reminded me.
Five selfish seconds of misspoken or heated words is all it takes to tear apart a thread of trust you helped weave with your friend over time. My friend deserves better than that. I don't always agree with my friend, but they have never betrayed me. And while I have never betrayed them either, I cannot truthfully deny that I was not a bit of an impetuous jerk about them. The truth found me out and I have no words other than to say that I am sorry; I'm stupid. And I ask for forgiveness, because that's the only thing one is in a position to ask for at this point. They are that friend, and it's shameful that I could neglect such a relationship.
(Blackmailing doesn't work. I mean, it can, but not if you're trying to hold on to a friend. :p)

To those of you who have made the same mistake I did, I would urge you to repair that friendship as soon as possible. It is better to lose pointless pride and retain a friend than to pretend nothing happened and move on a little less happy than you were when you had your friend.
Take a long look in the mirror. You may feel sad when you do, but it will make you a better person overall.