I was day hiking a fishing trail and came across a couple of young guys that had backpacked in and set up camp right on one of my favorite fishing spots. I stopped and chatted with them for a few minutes and they were planning an extended stay. When I returned a week later, their camp had been washed out by a flash flood (I knew they had set up in a poor spot but figured they knew what they were doing... apparently not). I figured they must have abandoned their stuff in the middle of the night because the entire camp was still there, soaking wet tent, sleeping bags, fishing rods etc.). I left everything as it was, thinking they would return to claim their gear. I returned a couple of times over the next month and scavengers and the river took care of the gear.
On the flip side, I was on a backpacking trip with my kids when they were young teens and we had an apparent communication problem. I thought my son had gone and grabbed the bear bag but apparently he thought I had gotten it. We had left my bear bag up a pole and since we hiked out in one day, we didn't notice it was missing until we got home (600 miles away). I lost my stove, a gas can, my pot and some food utensils (as well as the bag). I always imagined it hanging there until the fall when nobody was around and a ranger would retrieve it.