I had to retrieve a pine round that got away and rolled down to a neighbor's yard & crashed noisily into the grill on their deck. Embarrassing, but these things happen. Rarely, thank goodness.
Was once removing a Ponderosa next to a guy's house on very steep ground. I piled up slash below my work zone just in case a round got away from me. One round did, and it took a bounce and cleared my slash "fence." It was truly frightening 'cause that thing was hurtling downhill toward a neighboring place. All I could think was--if there's someone sitting inside that thing could crash thru and kill them. But in the end that round came rolling to a stop in the drive. (Found out later the place was vacant, when I got called to work there.) I retrieved the round that I had figured might weigh 50 lbs, and weighed it at home: 75 lbs. With velocity racing downhill . . . can't imagine what it might have done.
But if you've never repaired a fence or similar, you haven't done much tree work. It doesn't happen often, but it happens.