https://www.polygon.com/fortnite/2018/7/25/17613784/fortnite-ghost-peeking-fixed-epic-patch

“Ghost peeking” in Fortnite has been around about as long as the game itself, and while Epic has tried to remedy the issue in the past, the latest attempt may have done the trick.

In earlier versions of Fortnite, players could stand in a certain location on a stair that would allow them to shoot over the ramp and hit a target in front of them without exposing any part of their body to the enemy. In other words, someone who ghost peeked could shoot without any risk at all.