Leyton Orient striker Jonathan Tehoue stunned Arsenal with a late equaliser as the League One side earned a lucrative FA Cup replay after their fifth round tie ended in a 1-1 draw on Sunday.
Arsenal had beaten Barcelona -- widely regarded as the world's best team -- on Wednesday, but just four days later they were brought back down to earth by Orient's far less famous group of journeymen at Brisbane Road.
Tomas Rosicky headed Arsenal in front early in the second half to record his first goal in 13 months but that was not enough thanks to the late heroics from French substitute Tehoue.
The two London sides will have to try again at the Emirates Stadium on Wednesday, March 2, with the winners due to travel to Manchester United in the quarter-finals.
Arsenal manager Arsene Wenger made 10 changes from the side that had come from behind to beat Barcelona in the Champions League.
Most notable was the return of goalkeeper Manuel Almunia, with 18-year-old Spanish defender Ignasi Miquel handed his first-team debut.
Inevitably Orient kicked off as underdogs but the League One (third-tier) side were playing their seventh game in the tournament and had not been beaten in 11 games in all competitions.
Stephen Dawson, their captain, almost caught out Almunia with a long-ranger in the very first minute and, after Marouane Chamakh had seen a header and a shot saved at the other end, Charlie Daniels was only just over with another effort from distance.
Nicklas Bendtner fired a shot well wide before Chamakh wasted the best chance of the half by getting a poor touch to Kieran Gibbs' ball across the six-yard box.
Arsenal needed just seven minutes after the restart to take the lead, however, with Bendtner setting up Rosicky for a header that flashed just beyond the reach of goalkeeper Jamie Jones.
Orient, who had beaten Championship (second-tier) sides Swansea and Norwich to get this far, refused to give up and forced a number of corners, with Alex Revell also seeing a header saved by Almunia.
Dawson was still urging his side forward and Alex Song was shown a yellow card for a late challenge on his fellow midfielder.
Jones then kept Orient in the tie with a double save to deny Rosicky.
The Czech got on the end of Andrey Arshavin's clever pull-back but Jones parried, and then got behind the Russian's follow-up header.
The O's went close at the other end when Tehoue got the better of Song down the right and Revell fired narrowly wide from the resulting cross.
The same pair combined again moments later and Revell's shot smashed into Sebastien Squillaci.
Arshavin came close to wrapping up the tie with a shot from just inside the area that clipped the post.
But in the 89th minute Tehoue turned the tie on its head with a low drive that beat Almunia's poor attempted save and sent the Orient fans wild.