Emile Smith Rowe gave Arsenal the lead against Manchester United in bizarre circumstances in Thursday's Premier League contest, firing home with goalkeeper David de Gea down injured.
United shot-stopper De Gea was caught by team-mate Fred during an Arsenal corner and went down clutching his foot.
Harry Maguire headed the set-piece as far as Mohamed Elneny, who helped the ball into the path of Smith Rowe to half-volley past a still grounded De Gea from 20 yards.
After some initial confusion over whether the goal would be allowed to stand, referee Martin Atkinson pointed to the centre-circle to signal the start of Arsenal's celebrations.
United players were furious and caretaker boss Michael Carrick looked bemused on the touchline as opposite number Mikel Arteta punched the air.
With that strike after 13 minutes, Smith Rowe (21 years and 127 days) became the second-youngest scorer for Arsenal in a Premier League game at Old Trafford after Nicolas Anelka in February 1999 (19 years and 340 days).
United had shipped nine successive Premier League goals at Old Trafford without scoring, their longest such run, before Bruno Fernandes slotted in an equaliser shortly before half-time.