They say that a man is only as good as his word.
Most mistakes can be rectified but words cannot be taken back. What is said is said. You can apologise but you can't change the past.
What's worse than that?
I say email.
Email isn't just words that cannot be taken back, it's words that can forwarded. It can be used as "proof" that you said this and that.
That's another reason why I prefer to go and talk to someone about an issue rather than to send an email.