The Gunners arrived at Brisbane Road on Sunday as overwhelming favourites to book a safe passage into the quarter-finals of the FA Cup.
They will, however, have to see off Orient at Emirates Stadium in a replay if they are to face off against Premier League title rivals Manchester United in the last eight.
give them credit, 'cuz Orient remembered that this was still a London derby, even if it took until the last 20 minutes of the second half for them to do so.