Emi Buendia scored with the last kick of the game as Aston Villa stunned Arsenal to move three points behind the Premier League leaders.
Leandro Trossard’s sixth goal of the season looked to have earned a point for Arsenal after he cancelled out Matty Cash’s opener.
But Buendia struck in the 94th minute as Unai Emery’s side moved into second in the table.
Arsenal, who blew Bayern Munich and Tottenham away last month, suffered their first defeat in the league since losing 1-0 at Liverpool in August.
Villa Park erupted after substitute Buendia scored to give Villa their seventh straight game in all competitions. They have won nine of their past 10 in the league.