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A mysterious figure, originally thought to be Jacob, was seen for a split second by Locke in Jacob's cabin on Day 89 ("The Man Behind the Curtain"). Another figure, possibly the same one, was later seen by Hurley, two days later. ("The Beginning of the End")

Day 89 (Season 3)

Benjamin Linus took John Locke to see Jacob at a cabin deep in the jungle. Before arriving at the cabin, Locke discovered a ring of ash surrounding the perimeter. This ash appeared identical to that used by the Others to protect the Temple from the Man in Black in Season 6. ("The Man Behind the Curtain")  ("LA X, Part 2")

On entering, Ben and Locke found the room initially appeared empty, but as Locke prepared to leave the Cabin, he heard a voice whisper, "Help me." Believing the voice to be a deception on Ben's part, Locke turned on his flashlight. Immediately the cabin began to shake, jars of liquid flew across the room and the lamp was knocked over and set aflame. Ben grabbed an apparently empty rocking chair, telling "Jacob" that he had had his fun and needed to stop, before he was thrown against the wall by a mysterious force. In looking back across the room, Locke briefly observed a shadowy figure with shaggy hair and tattered clothing sitting in the chair. ("The Man Behind the Curtain") Ben later confessed that he had never seen Jacob in all his time as leader of the Others. He explained to Locke that he had been putting on an elaborate deception at the cabin but was just as surprised as Locke when the chaos erupted. ("The Incident, Part 1")

Day 91 (Season 4)

While hiking to the barracks with Locke's group, Hurley was separated and stumbled across the cabin. The door opened slowly, apparently of its own accord, and Hurley stepped onto the porch and looked in the window. Christian Shephard could clearly be seen sitting in the chair, still wearing his suit and white sneakers. Presumably, this was the Man in Black in Christian's form. Suddenly, a second figure appeared at the window, only their eye visible, staring at Hurley. The identity of this second figure is unknown. It is unlikely this was also the Man in Black, since he was visible as Christian in the cabin at the same time. ("The Beginning of the End")