Jose Mourinho's Chelsea looked set to coast through against League One Bradford City at Stamford Bridge as they opened up a two-goal lead in the first half through Gary Cahill and Ramires.
However, Jonathan Stead replied prior to the interval and a truly sensational turnaround followed as Filipe Morais, Andy Halliday and Mark Yeates all struck late goals to earn Bradford a 4-2 triumph that ranks among the greatest FA Cup shocks.
Chelsea had won their previous 12 home games, but their hopes of pulling off an unprecedented quadruple are now over following a second-half collapse that is unlikely to be forgotten in a hurry.
The result sparked understandable jubilation among the 6,000 travelling fans, while Mourinho entered Bradford's dressing room after the match to congratulate the players of the third-tier side.
Bradford's win overshadowed an outstanding victory for Middlesbrough, who knocked out Manchester City courtesy of second-half goals from Patrick Bamford and Kike.
City, FA Cup winners in 2011 and finalists in 2013, have now exited the competition to Championship opposition in successive seasons, having lost to Wigan Athletic in last term's quarter-finals.
Swansea City were another top-flight side to bow out on Saturday, with the early dismissal of Kyle Bartley a hindrance as the Welsh club lost 3-1 at Blackburn Rovers.
Bartley received a straight red card after six minutes, having been judged to be the last man as he brought down Josh King, but Swansea nevertheless took the lead through a powerful 25-yarder from Gylfi Sigurdsson.
However, Blackburn were able to make their extra man count thereafter, Chris Taylor, Rudy Gestede and Craig Conway finding the net before Sigurdsson saw red late on to ensure Swansea finished the match with nine men.
Tottenham fell victim to a late fightback by Leicester City, who claimed a 2-1 win at White Hart Lane courtesy of late goals from Leonardo Ulloa and Jeffrey Schlupp, and Liverpool face a replay with Bolton Wanderers after a goalless draw at Anfield.
Crystal Palace twice come from behind to beat Southampton 3-2, Marouane Chamakh scoring twice as all five goals came in the first half at St Mary's Stadium.
Victor Anichebe's brace fired West Brom to a 2-1 win at Birmingham City and extended Tony Pulis' unbeaten start in charge of his new club, while Reading won 2-1 at Cardiff City and Derby ran out 2-0 winners over Chesterfield.
There will be a replay between Sunderland and Fulham, who failed to find a goal at the Stadium of Light. Preston North End and Sheffield United drew 1-1 and will also meet again.
