It wasn't shot over the wall, but to the side of it. The issue is that the arrow's in-game model, which is used for detecting collision with stationary things like walls and non-player entities (pots, trucks, statuary) passed within an inch of the side of the obstructing wall, but the arrow's "hitbox" - a defined-but-invisible 3D polygonal area overlayed on the visible model - is so much larger than the arrow that it intersected Widowmaker's head's "hitbox." Blizzard has admitted that they enlarged Hanzo's arrow hitboxes to artificially boost his accuracy.
IMO if they can nerf Torbjorn's turrets on console only, they can nerf Hanzo's arrow hitboxes on PC only.
Actually, I might be misremembering, but I think something like that actually happened last night. Though, Terrik didn't jump into the pit, just went to the edge of it so he was visible to Dei, who then could fly back up to him.