Even if I had no personal experience, I've seen plenty of evidence of "karma"/reaping what you sow/what goes around comes around.
(mitigated by the "rain falls on both the righteous and the wicked")
But you were asking for personal stories, so here goes:
About a year or so after I graduated from high school, I was at a party hosted by a friend. He had asked the band I was in to play.
We were in the middle of the first set when a small group of guys from another high school barnstormed the place with a bad attitude.
Frustrated that the party was in danger of breaking up (sucks to have to break down and load the equipment after playing only a half set!), I stuck my nose in the middle of it.
Well, one of those guys called me out, and I had been drinking and nobody from our group intervened, so I stepped outside with him.
About this time I noticed that he was bigger and stronger... oops.
But I slipped his punch, got a good lick in and he spun to the ground. Nevertheless, I wasn't sure if things would continue to stay in my favor if I let him get up, so I told him "You better not get up!"
He rolled over, got up on his hands and knees, so I kicked him in the face. Now, I had never studied any martial arts at this point in my life, but I had played soccer for many years...
The kick lifted him off his hands and laid him on his back. I told him, "Just stay down!" but he started to get up again. Now I was REALLY concerned for my safety, so I kicked him in the face a bit harder.
He started to get up AGAIN and again I told him "DON'T GET UP!" He still tried to get to his feet, so I kicked him AGAIN in the face, this time even harder. It laid him out on his back and he was unconcious.
I went in the house to cool off for a bit, came back out five minutes later: he was still out cold. Now I was a bit concerned for HIS well being.
Another couple of minutes later he finally came to — and immediately started coming at me. But both his friends and my friends were convinced he had had enough — his friends pulled him back and into their vehicle, my friends pulled me back into the house.
Several years later, I was working at a gas station and some guy came up to the kiosk window, bought a pack of cigs, and said, "You remember me?"
"Nope," I said.
"You broke my jaw a couple of years ago — broke it in three places. You really messed me up," he said.
I shrugged. I felt bad about what I had done even before I heard his jaw had been broken, but now I was feeling REALLY bad and also wondering if I was about to be served papers for a lawsuit.
"I just wanted to thank you," he said.
I blinked.
"I'm not being a smart ***," he said. "I mean it. Before that fight I was a real *******. Plus, I was overweight and couldn't get a girlfriend. After that fight, I had my jaw wired shut for six weeks, lost a bunch of weight, plus it really mellowed me out. Now look at me: I'm lean, I get chicks all the time, and I'm a helluva lot nice. Thanks — you really made a difference in my life."
"Er.... glad I could help," I said, completely blown away by what he said.
About three or four years later, I had a crazy girlfriend who decided to go off and "party" with three other guys. Rather than simply writing her off and moving on, I thought it would be a good idea to talk to her as they left the bar.
They objected to me interrupting their party and as I told them to butt out, someone sucker punched me. I turned to look at him, and another guy sucker punched me from another direction. Funny thing was, neither of
those guys were even with the ones who were "partying" with my wayward girlfriend. They had just been drinking and thought it would be fun to punch someone in the face.
I wasn't happy with how things were going, so I grabbed one of the three party boys by the lapels, stepped behind him with my right leg and dropped his shoulders and head to the gravel parking lot. Then I reached down and grabbed his windpipe and said, "Anybody touches me again, and I swear to god I will rip this throat out."
About this time, the local police pulled up on the lot.
"What's going on here?" they asked.
"Nothing — we were all just talking," they said.
"Well, I suggest you all break it up and go somewhere else," they instructed.
We all hopped into our vehicles and I followed them to another bar so I could demand my "girlfriend" to talk things over.
This time, however, I thought it would be a good idea to slip the escrima sticks my friend had lent me up my sleeves "just in case" they decided to jump on me again.
While I didn't have any formal training, he has showed me a couple things with escrima sticks and I figured they might even the odds some.
So as I asked her to come talk to me, they turned around and said, "Oh, its YOU again" and started moving toward me.
I dropped the sticks out of my sleeves into my hands and prepared to engage.
One of the three guys, who I later learned was named "Jimmy" and was a VERY rough character, said, "Oh yea? I've got just the thing for YOU."
He went to his car, reached in and pulled out a tire tool. They spread out and began to feint in. I would take a couple swings, and that fellow would back up while another feinted in.
Then the guy on my right — the same one I threw down into the gravel at the other bar — shot in for a "shoot" takedown. I backpedaled off at an angle, hammering him in the head, face and shoulders with a combination of about six to seven strikes.
His shoot became a sprawl face down onto the blacktop parking lot.
"Oh my god," I thought. "Did I just kill this guy?"
His two buddies helped him to his feet, to my immediate relief.
They turned and looked at me and immediately all rushed me at the same time.
They grabbed my arms, pulled the sticks out of my hands and threw them, and pulled me to the ground.
Jimmy then proceeded to drop the tire tool down on my skull. I wrenched one arm free and managed to fend of a couple blows.
One of the holders re-grabbed my arm and held me more firmly.
Jimmy continued to beat me in the head for a couple of strikes while I struggled. Then one came down right on my jaw.
I knew THAT one had hurt me: I could feel a couple of teeth loose in my mouth. At that moment, for the first time in my life during a physical encounter, I completely lost the will to fight. I just wanted to go home and it occurred to me that I might never ever go home again.
While my life didn't "pass before my eyes," I did realize at that moment that this could really be the end of my life, and I regretted the grief my parents and other family would experience at my untimely passing.
Ironically, I could see the red and blue flashing of a police cruiser's lights that was right around the corner of the bar. So close, and yet so far away.
And Jimmy was still hitting me in the head with the tire tool, despite cutting his hand on the "prybar" end he was holding as the curve of the lugnet end was what was making contact with me.
About that time Scott Campbell walked up. He and a few other had been watching the fight through the glass side-exit of the bar.
"Hey," he said. "Don't you think he's had enough? You are going to kill him."
Jimmy reached down and grabbed me by the face ("OUCH!" I thought, "That HURTS!) and said,
"If you ever mess with us again, we will KILL you — do you understand me?" he said.
I knew it wasn't just an expression. I nodded enthusiastically.
They walked back to their vehicle and I climbed to my feet.
I jogged over to the police cruiser, knocked on the window, and showed the startled officer the three bloody teeth I had spit in my hand.
"Those guys just knocked me teeth out — get them!," I slurred through my broken jaw, pointing to the vehicle pulling off the lot.
"Do you need an ambulance?" he asked, visibly shocked. He had been filling out a citation as I knocked on the window.
"NO!" I said. "I want you to GET THEM. Hurry — they are getting away!"
"Are you sure you don't want an ambulance," he asked.
"Aw, forget it," I thought. I jumped in my car and drove home.
I rushed into the bathroom to look over the damage. It was bad. There was a hole in my lip large enough to stick my thumb through and my jaw was visibly mis-shapen.
It occurred to me that the ladies probably wouldn't find me attractive any more, and I began to tremble as I made my way to my parents bedroom door. Shock was beginning to set in.
It's too late to make a long story short
but there was no way to "save" my teeth: my jaw was more than just "broken" where the tire tool had hit it. The maxillofacial surgeon said that part of my jaw "looked like oatmeal."
I spent the next five weeks with my jaw wired shut: apparently I heal a bit quicker than average. Once my jaw healed enough, the Good Doctor put implants into my jaw upon which he mounted a permanent bridge.
During those five weeks, I decided I would like to learn a bit more about martial arts. My friend and I started training regularly for a couple weeks before we decided to join the local Moo Sul Kwan for formal training.
While that is another story, let me wrap things up by saying it proved to be a life-changing, character-building experience for me, as well. I generally began making better choices about my female companions and the types of situations I put myself into from that point on.
My face healed up pretty well, although there is a visible scar on lip if you look for it, but is not generally noticeable and my jaw line appears to be straight.
All in all, it may have been the best thing that ever happened to me.
While I never "thanked" Jimmy for working me over, I did run into him once more before he died.
I was talked into moonlighting as "chief of security" for a couple of months at a bar after several years of martial arts training. I think I was a brown belt at the time.
I am proud to say I NEVER had to get physical with anyone during that job, including the time Jimmy showed up at the place and had an argument with his ex-girlfriend who was a waitress there.
The boss told me it was time to ask Jimmy to leave, so I did exactly that. I politely advised him the boss had asked him to leave.
"No problem," he said, and left without incident.
A couple of months after that, the boyfriend of husband of a girl he was seeing shot him like six times in the chest. He survived.
A few months, maybe a year, later he died in a somewhat mysterious car accident — his car left the roadway on a turn and Jimmy died in the accident.
They say what goes around comes around, that you reap what you sow. I believe it.
How we treat people will eventually turn back to us. Hence also the "golden rule" of "do unto others as you would have them do unto you". It's evident here on Martial Talk as we all interact with each other through our posts and threads. Post bad stuff and you're gonna get bad replies (and reps) and vice-versa. 
