Jack Grimmer netted a 97th-minute winner as Shrewsbury Town staged a stunning comeback to triumph 3-2 and knock Championship promotion hopefuls Sheffield Wednesday out of the FA Cup fourth round.
League One strugglers Shrewsbury twice trailed to goals from Lewis McGugan either side of Jean-Louis Akpa-Akpro's equaliser and were still behind with three minutes to go.
Shaun Whalley levelled by converting the rebound after Akpa-Akpro's penalty was saved and deep into stoppage time, Grimmer headed home to spark jubilant scenes and send the Shrews into the fifth round for the first time since 1991.
"People are asking for attacking football. I cannot do any more than my right-back getting on the end of a cross in the 97th minute!" said Shrewsbury boss Micky Mellon.
Saido Berahino had twice given the home side the lead, his second coming in the 84th minute, but Jon Taylor equalised to earn the Posh a replay.
Arsenal, winners in each of the past two seasons, recorded their 14th consecutive FA Cup victory as Alexis Sanchez inspired Arsene Wenger's men to a 2-1 home win over Burnley.
Sanchez, who had not started for two months, set up the first for Calum Chambers and then netted the winner after Sam Vokes had levelled for the visitors.
Nacer Chadli claimed a brace in Spurs' 4-1 win at Colchester United, while Kelechi Iheanacho's hat-trick inspired City to a 4-0 thumping of struggling Aston Villa.
Odion Ighalo's 89th-minute goal saw Watford defeat Nottingham Forest 1-0, while Palace overcame Stoke City by the same scoreline and Bournemouth came from behind to see off south-coast rivals Portsmouth 2-1.
