In the 1900 Olympics the Dutch rowing team were short a cox. They used a rower in the semifinal, Hermanus Brockmann, but decided his 60kg weight was too much of a handicap.
So the rowers, Françoise Brandt and Roelof Klein, picked a ten year old French boy (25kg) out of the crowd and asked him to cox for them.
They won the gold. And took a photo with the boy. But his identity has never been established.