My grandmother used to have a fridge magnet that said, "We get too soon old, and too late smart." In the past decade I have gotten a lot older, and a little wiser.
I've learned something about people: the way they treat others, is the way they will eventually treat you. If someone you know is lying to other people, don't think, "Oh, but we're friends. He won't lie to me!" Of course he will, and probably already is. Or if a friend is always talking about others behind their backs, count on it that she is talking about you too.
Credibility, once lost, is almost impossible to restore. Everyone tells a fib or two, now and then. But there is a line. A friend of mine posted a resume which is about 80% fictional. The 20% which is true might as well be fictional too, because no-one who knows him will believe even that.
It's amazing how self-aware most people are not. Especially those who have really bad characters. But almost everyone is blind to their own character flaws, minor or major. (I, of course, am the exception...)