One penalty stroke, called by a player on himself, for an infraction that no other human being observed. It cost him the 1925 US Open by exactly that margin.
The peculiar structure of golf
Almost every sport employs officials whose job is to detect breaches. Golf does not, for most of the course. It hands the responsibility to the competitor and assumes it will be discharged.
This works about as well as you would expect, which is why the cases where it visibly works are worth recording.
Worcester, 1925
In the opening round at Worcester Country Club, Jones played from thick rough. Addressing the ball, he caused it to move fractionally.
His playing partner did not see it. The gallery did not see it. No official was in a position to see it.
Jones announced the penalty. USGA officials questioned whether he could be sure and suggested he need not take it. He insisted, and the stroke was added.
The margin
He finished the seventy-two holes tied with Willie Macfarlane. The play-off was over thirty-six holes and Macfarlane won by a single stroke.
The penalty and the losing margin were the same number.
What he said about it
The press attempted to canonise him. His response is one of the most quoted lines in the sport, to the effect that praising him for it made as much sense as praising a man for not robbing a bank.
He repeated the behaviour at the 1926 US Open, which is the detail that turns an anecdote into a description of character.
The rest of the career
Jones never turned professional. He qualified as a lawyer, held degrees in engineering and English literature, and played competitive golf alongside all of it.
In 1930 he won the Open Championship, the US Open, the Amateur Championship and the US Amateur in a single season, a sweep nobody has repeated. He retired from competition at twenty-eight and later co-founded Augusta National.
The USGA's sportsmanship award carries his name.
Why it goes on a wall
Because the only witness was the person who stood to lose, and he reported it anyway, and then refused to be congratulated for it.
One stroke, and the play-off went by one.