Australian Matt Jones has ended the second round just one stroke behind leader Brian Harman at the PGA Tour's Travellers Championship event in Connecticut.
Jones is on nine-under alongside Americans Zach Johnson and Russell Henley.
But it was Johnson who emerged as the talk of the day after he fell foul of the "10-second rule."
Johnson's attempted birdie putt at the par-four third hole stopped right on the edge of the cup, but frustratingly did not drop - at least not for a while.
A player in such a situation is "allowed enough time to reach the hole without unreasonable delay and an additional 10 seconds to determine whether the ball is at rest".
After walking to the ball and then waiting his allotted time, Johnson prepared to tap in for par when it toppled in of its own volition.
He summoned a rules official and was informed he had recorded a par four, even though he had only struck the ball three times.
The relevant rule requires a penalty stroke in such situations if the 10-second wait has expired before the ball topples in.
"I probably should have really walked slow up there and taken my time. It's a rule protection for pace of play," Johnson told the Golf Channel after carding a two-under-par 68.
"They said it took about 15 seconds to fall in, so I was about five seconds over."
Johnson, without a win since the 2015 British Open at St Andrews, was happy with his position as he seeks to end his nearly three-year drought.
"Got an opportunity for the weekend," he said. "I love what we're doing and how we're doing it, a lot of fairways, a lot of greens."
Leader Harman had three bogeys on his outward nine, but was more assured coming home to post a 130 halfway total.
"I'm really happy with how I'm playing," the two-time PGA Tour winner said.
"I've played pretty aggressively. I'm just going to keep on doing what I've been doing and see what happens."
Rory McIlroy, a stroke from the overnight lead, carded a 69 to slip three strokes back.
"I need to do a little bit of work on the range this afternoon and get more comfortable for the weekend," the Northern Irishman said.
Defending champion Jordan Spieth, who shared the first-round lead with Johnson, backtracked with a 73 to plunge six strokes off the pace.
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