After beating Barcelona in midweek Arsenal was brought down to earth by third-tier opponent Leyton Orient, which held the Premier League club to a 1-1 draw in the FA Cup fifth round on Sunday.
Orient substitute Jonathan Tehoue powered past Arsenal defenders Kieran Gibbs and Ignasi Miquel before striking under goalkeeper Manuel Almunia in the 89th minute to cancel out Tomas Rosicky's header.
Arsenal, which beat Barcelona 2-1 in the Champions League on Wednesday, will now have to play a replay against Orient. The winner will face Manchester United in the quarterfinals.
Manchester City secured a more comfortable victory over another League One side Sunday, routing Notts County 5-0 in a delayed fourth-round replay.