The favourite, Epirus, ridden by Bill Scott, stumbled into a ditch on the inside of the gravel road which crossed the Doncaster track around 200 yards from the start, and outsider The Prime Warden hit Scott as he stood up.
Shortly after, a greyhound ran onto the track, bringing down another two horses - Dardanelles and Henriade.
Having survived these two fracas, and with only four left in the race, Day won on Mango, although not without beating the horse severely.
Elder brother John, riding Henriade, maintained he would have won if not for the greyhound.
He was carried to a house at nearby Longstock, but died without regaining consciousness on 14 March.