Kevin Kisner was set to take the outright clubhouse lead at the halfway stage of the British Open before a double-bogey at the 18th at Carnoustie forced him back alongside Zach Johnson at six under par.
Kisner, who led overnight after a first-round 66, was three-under for his round heading to the last tee.
But the American's approach shot found the Barry Burn, scene of Frenchman Jean Van de Velde's infamous meltdown on the last day in 1999, forcing him to take a drop before two-putting for a one-under-par round of 70.
Johnson, the 2015 Open champion, had earlier shot a 67 in wet conditions that contrasted with the fine weather later in the afternoon on Scotland's east coast.
South Africa's Zander Lombard, a late starter, was also six-under for the tournament after grabbing his second birdie of the day at the seventh hole, while American Pat Perez moved to six-under after a birdie at the long 14th.
England's Tommy Fleetwood is in the clubhouse on five-under after a superb round of 65 having gone out in one of the first groups.
Rory McIlroy, the Open champion in 2014, shot a second successive 69 to sit at four-under at the halfway stage, level with South Africa's Erik van Rooyen.
Kevin Kisner watches his shot from the third tee during his second round at Carnoustie