Counter-Strike is a game where the meta hardly ever changes, and once it does it is almost always due to new strategical approaches and grenade usage. New weapons are hardly ever implemented, which has meant the reign of the M4A4 and AK-47 for almost the entirety of the game's life cycle. However, in 2015 Valve implemented a new gun that changed every aspect of Counter-Strike as we knew it.
I am, of course, talking about the R8 or as the player base called it the "Revolver". In a new patch, the weapon was added alongside updates to sound and spray patterns, and it quickly shocked the entire community. With the ability to one-shot your opponent with a single shot to the chest it became the most overpowered weapon in Counter-Strike history, only costing $850.
Take a look at the short but amusing story of the most broken gun in Counter-Strike history:
Whilst the lifespan of the R8 revolver only lasted 48 hours it was a clear-cut indicator of how fragile the Counter-Strike foundations are, and how a single new weapon can abrupt the entire CS:GO scene. The gun was quickly nerfed to the ground and is now only used by players and pros for fun. Most notable is the Swedish legend "JW" and his usage of the gun after its nerf.