Russell Henley went from contending to likely missing the cut Saturday at the Mayakoba Golf Classic thanks to an unfortunate and costly mistake.

Henley was signing balls for fans after his second round when he realized one of the balls was slightly different from the one he uses. Henley said he was unsure how the ball got in his bag but that he used it on hole Nos. 9-12.

As a result, Henley was penalized two shots on each hole, eight strokes in all, for violating Rule 20-3, which covers all rules not laid out in the Rules of Golf.

The specific rule that was breached was the One Ball Rule, which the PGA Tour and most pro and high-level amateur events recognize. In regards to the One Ball Rule, the USGA states: "When changing balls, the player is permitted to substitute a ball of another brand or type unless the committee has adopted the One Ball Condition of Competition (see Appendix I; Part C; Section 1c)."

Henley ended up with a 6-over 77, which dropped him to 1 over. Instead of entering the third round at 7 under and in contention for a possible victory, Henley will likely miss the cut.