Here's one of the more exciting, fully exploitable glitches in Fortnite's Battle Royale mode. It lets you move and shoot other players from under the grassy floor -- and be completely invisible and invincible to them while doing it.
All you need to do is go to the location on the map above (indicated with a red circle) and get inside a small hut that rests under the big tree. Get inside the hut and break the floor using your pickaxe.
When the floor is broken, you will see that the walls of the hut stretch down into the ground. Break those lower parts of the walls, too. You will then see an opening that will allow you to move underground and basically do whatever you want without getting detected.