Perhaps the most interesting incident in this O. Henry story is the painting of the last leaf itself. As a final gesture of kindness, the old man downstairs paints a fake ivy leaf in the place of the actual, living, green leaf itself. He does this so that Johnsie will live, as she has said that she will die from her pneumonia when the "last leaf falls."
As a result of the kindness, the old man dies of pneumonia himself, having painted his "masterpiece" in the worst weather possible. Ironically, Johnsie is inspired to live by the leaf's presence, and recovers. This twist is a classic example of O. Henry's use of bittersweet irony to resolve his stories' plots.