Rory McIlroy didn't realize his biggest misstep in the first round of the Pebble Beach Pro-Am until after the 18th hole.
An accidental illegal drop on his 16th hole (Spyglass Hill's seventh hole) saw McIlroy take a two-stroke penalty, going from 3-under on the day and five strokes back from Patrick Cantlay to 1-under.
At issue was a USGA rule change on back-on-the-line relief that went into effect last year. Previously, if you hit a ball into a penalty area, you could use back-on-the-line relief, which allows you to take a one-stroke penalty and drop the ball along the line created between the hole and where your ball entered the penalty area.
Before 2023, you could make that drop within one club length of the line. The rule change modified the rule so you could only drop the ball on that line.
McIlroy used the old version of that rule. When his drive went wide and landed in the penalty area, he used back-on-the-line relief, then dropped the ball a club length to the right. The result was a bogey turning into a triple bogey.
Here's how PGA Tour rules judge Mark Dusbabek explained it on the broadcast:
“When the rules changed in 2023, back-of-the-line relief — you still go back of the line in line with the flagstick from where the ball is. However, you have to drop it on that line and it can roll in any direction, even forward, one club length. Rory from that line dropped one club length to the side, which makes it a wrong place. And it’d be a two-stroke penalty."
"Rory was met in scoring by our chief referee over there, Steven Cox, and he totally owned the mistake. Very professional about it and admitted to the rules mistake.”
McIlroy publicly owned the mistake as well after the round, saying he wasn't aware of the rule change, via Ron Mintz:
"Yeah, yeah. So I took an unplayable on 7 and I took it back online. Then unbeknownst to me the rule changed in January 2023 where you used to be able to come back online, take a club length either side. That was changed in 2019 to be able to do that. I wasn't aware that that rule was changed again in 2023, so I took a drop thinking of the 2019 rules when everything was sort of changed not knowing that the rule was changed again in 2023, so got a two-stroke penalty there."
The mistake was part of a rough end for a promising round. Through 14 holes, McIlroy was 6-under without a bogey on the day, then posted three straight bogeys before finishing the round with a par.
Nick Bromberg contributed to this report.