Despite being forced to field a side lacking a single recognised striker, Darlington shocked Posh when snatching the lead after just a couple of minutes. Ricky Ravenhill's low shot took a deflection off home defender Craig Morgan and flashed past unsighted goalkeeper Joe Lewis.
Darlington were then happy to drop back and defend, skipper Alan White showing the way when getting an Aaron McLean effort off the goal line.
But the Quakers lost goalkeeper David Stockdale, who tried to battle on after taking a kick as McLean chased a through ball.
Stockdale finally had to leave the field following a second spell of treatment but replacement Przemyslaw Kazimierczak quickly made his mark, dealing with a George Boyd effort and then stretched to his left to pull down a a header from Craig Mackail-Smith.
Things became even worse for muted Posh six minutes after the break when Darlington doubled their lead.
Micky Cummins was not picked up and was unchallenged as he comfortably beat Lewis after meeting a cross from Rob
Purdie who had dispossed Adam Newton.