Rule 10.2b(4) says:

(4) Restriction on Caddie Standing Behind Player. When a player begins taking a stance for the stroke and until the stroke is made:

  • The player’s caddie must not deliberately stand in a location on or close to an extension of the line of play behind the ball for any reason.

Well, someone didn’t get then message, and was penalize 58 strokes over parts of two rounds for repeatedly breaching the rule:

The USGA issued a clarification on the Rule earlier in the year which essentially reversed the second bullet point about how a player can not “erase” the caddie being on an extension of the line if he backs out of his stance if he’s off the putting green. Players may reset the status of this if they back off and re-take their stance legally anywhere on the course now.