Phil Mickelson maintained his four stroke lead at the midway point of the Waste Management Phoenix Open but for the second day running he missed out on a notable achievement on the final hole.
Mickelson's six under 65 gave him a 36-hole total of 125, 17 under par and four clear of fellow American Bill Haas.
But a par at the last would have seen the left-hander set a new US PGA Tour 36-hole record of 123, bettering the record of 124 shot by Pat Perez at the 2009 Bob Hope Classic and David Toms at the 2011 Colonial Invitational.
Instead, he found the water from the tee and dropped shots for the first time this week, carding a double-bogey six.
The disappointment came hot on the heels of another narrowly-missed scoring milestone, when Mickelson's birdie putt on the ninth 24 hours earlier, which would have given him a 59, lipped out in agonising fashion.
Unlike then though, when he said he was "mortified", today he could see the upside.
"I think it's very possible that's going to help me because it's got me refocused," he said on www.pgatour.com. "I cannot ease up on a single shot.
"These guys are going to make a lot of birdies and I've got to get after it and cannot make those kinds of mistakes.
"Hopefully it'll help me refocus for tomorrow's round and come out and shoot something low."
At the head of the chasing pack is Haas, who carded eight birdies in a blemish-free 64 to sit 13 under, a shot ahead of Brandt Snedeker and Keegan Bradley, who carded an eagle, seven birdies and a bogey in his best-of-the-day 63.
Three-time Major winner Padraig Harrington slipped down the pecking order to joint 20th on eight under after he could manage only a one under 70.
Defending champion Kyle Stanley just made the cut - he was bang on the mark at four under.