Not the best advice, but a pretty valuable and memorable lesson.
When I was 17 yrs. old, I worked for a messenger service in NYC as a team of 2 with this other guy who was from the Midwest somewhere. He drove, I delivered. He had some years on me, and I looked up to him because I knew he was crazy, fearless, and I believed him to be a pretty intelligent dude. He was a biker type, I was a skinny "new wave" kid, but we hit it off really well and became pretty good friends. We had an incident once that I won't write about with some messed up drug dealers, and he took care of it like a scene out of a movie. Without even getting the least bit nervous... was like something just shifted (the wrong way) inside him. Anyhow, I had a lot of respect for the dude. So.....
Something happened outside of work with me one day where I wound up with a black eye. When I went back to work and he saw it, I made up a story about how it happened because, well, I didn't want to tell him the real story

. About 2 weeks later a friend of mine joined us at work, and the driver brought up my black eye incident. My friend laughed and said what really happened. That didn't go over well. I almost lost my life.
The guy pulled the van over, plastered me against my seat and while I forget the exact words... said he should kill me, for lying to him. The fear isn't what got me so much as the realization that this guy trusted me, and I broke his trust. He obviously had a sore spot for people who lied, but he gave me a good 10 minute lecture on how he thought we were buddies, and if 2 people are buddies there's never any reason whatsoever to lie to each other. He was convincing, because behind all his insanity was a whole lot of truth that made complete sense. I got it. We continued working with each other, but the friendship was pretty much over. It sucked, cuz we had a lot of fun up to that point. Anyhow, I thought very differently about lying from that day on. And thankfully I got out of that without a scratch.
So many lessons I've had like that... was the first one to pop into my mind.