Every once in a while, a golfer fumbles with their ball or their clubs or their ball marker on the putting green, and they accidentally move their ball while it's on the putting surface or move the marker while trying to replace the ball. Under the old rules of golf, that would be a penalty for the player, and it's cost pro golfers tournaments in the past.
However, under the new Rules of Golf starting in 2019, that penalty is no longer an issue.
If a player (or an opponent) accidentally moves their ball or ball marker on the putting green, the player is able to replace it without penalty to its original location before it was moved.
The big reason for this change was what happened to Dustin Johnson at the 2016 US Open. The greens have become so fast, while slopes remain severe, so a ball at rest could move on the putting green without the player actually causing it to move on purpose. Rather than penalizing a player for a slip of the hand or just plain physics, the ball can now be easily replaced without a problem. A ball is to be replaced from where it originally was if it wasn't moved by any clear agent other than wind or gravity.
However, a player isn't able to replace the ball if they accidentally move it while on the course -- with the notable exception of stumbling upon your ball and moving it while searching for it within the 3-minute window.
The good news is now players won't face penalties if they fumble around with their ball-marker or slip on a wet green and hit the ball.
The Rules of Golf 2019 go out of their way to correct some of the mistakes in the past that have led to harsh criticism of the USGA, and this is one of those cases.