https://www.pcgamer.com/black-ops-4-players-are-crouch-running-at-super-speed-thanks-to-a-glitch/

Players are using a Black Ops 4 glitch that lets you stack the same perk multiple times to gain super speed in multiplayer matches—and Treyarch is cracking down on anyone exploiting it.

The glitch involves a complicated process of joining and leaving custom lobbies, creating a specific custom class, using certain equipment and completing a sequence of in-game actions, but the upshot is that you can clone any perk, stacking the effects on your character. Players have been doing it with the Skulker perk—which normally lets you move a bit faster while crouching or prone—to create super-speedy soldiers that zip around the map at 100mph, legs bent.