I have hit my share of nails and barb wire as as much as anyone who does trees probably does. Worst was a 50" willow we were taking down in a from yard. We were making the bottom cut, about 12" up since the swell was real bad, and as we got nearly finished we started hitting something. 3 chains later and a couple trucks on rocks to yank the bottom 8' over the tree finally drops. Turns out the tree had split when it was 5 or 6 years old and someone had clamped the 2 halves together with a 10" bolt and some junky steel scrap for plates on the outside.
Best I have heard was a from a retired firewood cutter whom I did a ton of work for. Been 7 years so dont recall exactly, but apparently him, or possibly a friend he was working with, hit a gun in a tree! Their best guess the tree was a double leader and some hunter leaned the gun against it and couldnt find the gun. tree grew around it and next thing you know a guy with a chainsaw is cutting through it! Attempting to at least hehe. Not sure if it's true but a good story either way. I like to think it is. He also, personally I know, found a chainsaw bar stuck in a tree that had been there for a few weeks. Apparently the genius who left it didnt have a another saw, bar, money or friend to get a saw.