I've only really done it once, and it was a long time ago, but it is a funny story. Note that I would not do the same thing today.
Back during my Fresman year in college, 4 of us went out to the gravel pits east of USU. (there are houses there now). Back in those days, lots of people would go into the pits to plink. The area you drove through to get there often had many four wheelers.
On the way into the pit, we saw a Toyota 4x4 parked near the top. As we drove past, we waved at the driver. Our assumption was that he was out four wheeling. He saw us. Of that I'm sure.
We drove down into the pit, and parked. As I stepped out of the car there was this horrendous noise, and a rifle bullet passed so close to the roof of the car that it actually vibrated.
I hit the dirt.
So two of us are laying on the ground, wondering who was shooting at us, with two others still in the car. The guy from the Toyota walks to the lip overlooking the pit, scoped bolt action rifle in hand, and shouts down at us. "I was here first!"
The response from one of the guys with me is unprintable according to Art's Grandma. But let's just say this guy was lucky he didn't get shot.
So we got back in the car and drove out of the pit. Some of us contemplating murder and thinking of good places to hide a body. We drove past the moron, and back down the road. We stopped approximately 150 yards off to the side, and well behind the Toyota.
We were seething. But since we had come out here, we decided to wait until the moron was done, and then go about our shooting. So we watched while he shot at targets that he had set up in the pit. (perhaps 50 yards) He was a lousy shot. Which made him shooting right over our heads extra super offensive (not that I wasn't really really angry anyway).
So as we sit, there, getting angrier and angrier. My friend Seth gets out his 45-70. Seth is a bit of a hothead, so my first question is to make sure that he isn't going to just plug the jerk. Nope, since the Toyota guy is such a lousy shot, he just figured we should help him a little bit.
At seventeen years old, this strikes me as a great idea.
So we all got out our rifles and took up position, then opened fire on his targets. Shot the heck out of them.
He starts screaming at us. "Hey! I was here first!"
The response from my Ecuadorian roomate Enrique was priceless. "Yes! But we have more guns!" So we flipped him the bird and kept shooting his targets until he got angry and left. At close to 200 yards, we were getting a lot more hits than he was.
Like I said, not what I would do today, so don't feel the need to critique my actions from thirteen years ago. But it was rather entertaining.
Another good shooting somebody else's target story: The above mentioned Seth was in the Navy and was at one point on some sort of security detail for his ship because it had nukes. (sorry don't know what you Navy guys would call it, his regular job was an ET).
The security detail was just regular sailors with other jobs, but they got to have .45s occasionally. (unloaded, with a magazine in your pocket). The team needed a certain number of members so that they wouldn't have to do the security stuff for too many hours. There was one other sailor who wanted to join, but he was a horrible shot, and needed to shoot a certain score to qualify.
So during the qualification shoot, the security team was standing in a line, shooting at their targets. The sailor who couldn't shoot's target was mostly empty of holes. If I recall correctly, they needed 40 out of 50 to qualify. So once the other sailors got to their last magazine, they all emptied it into the last fellow's target.
He got the best score.