A few pics showing how much defect can be present in a tree before it fails. The first picture is of a larch on a woodland edge that was mostly dead, leaning heavily in the direction of the chainsaw. It was approximately 10 meters high with foliage on the leaning side. You can see that some parts of it have been dead for a while. Only the really light stuff on the bottom was alive.

The next two pictures are of a sycamore which managed to remain standing even after we disconnected it from the neighboring tree. There was very little sound wood left and yet it managed to support probably about 2 tons of tree.