Walt had his consciousness overwritten in the first episode of season 2 by Traveler 4112 and subsequently by Traveler 4991 after it was discovered that traveler 4112 was a member of The Faction.
Walt didn't technically have an 'adverted death'. He, among other people in the FBI, was sent a text to meet in the room where the Quantum Frame was being held. For a while, it was believed that the Quantum Frame had a bomb which would go off if anyone tried to destroy it, which later turned out to be untrue. In reality, the Faction had hijacked the Quantum Frame and were taking over the bodies of hosts who weren't destined to die.
4112 Future History
Traveler 4112 was a member of the Faction, fighting against control of the Director. Before the Director was shut down, his consciousness was sent through to the quantum frame and later to Walt Forbes' body.
Walt Forbes had been a Special Agent in the FBI with Grant MacLaren for some years prior to being overwritten.
Walt, along with other Faction members from the army, tries to stop Grant from leaving a nuclear weapon where the Director is destined to be built. He knew by doing this that the Faction's plan to control overpopulation with the virus would fail as it powered the Director long enough for him to send the cure and overwrite some of the Faction travelers. He failed and Grant chased him to the edge of a cliff where he was overwritten by traveler 4991. (U235)
After overwriting traveler 4112, he explains that what Grant and the rest of the team did was enough to power the Director for three seconds, in which he was able to transfer consciousness to overwrite him and Jenny (possibly among others) and send a cure for the disease which the Faction had spread. (U235)