Playing foursomes (alternate shot), Furyk's tee ball on the fourth hole flew into a very muddy area. It was ruled that Furyk/Fowler were entitled to a free drop due to abnormal ground conditions.
In alternate shot, it was Fowler who took the drop. But rather than lifting Furyk's ball out of the mud, as he should have, Fowler pulled a ball out of his pocket and dropped that one.
Nobody immediately noticed the error, and Fowler played his shot. Only then did Fowler's caddie realize the mistake. Had Fowler realized the mistake before playing the stroke, he could have re-done the drop properly without penalty. But the match play penalty for playing the incorrectly dropped ball was loss of hole, and that's what Furyk/Fowler were assessed. At the time, it gave Westwood/Kaymer a 2-up lead.
Furyk and Fowler fought back, however, and on the 18th green Fowler sank a testy downhill putt of about 4-5 feet for birdie, halving the match.