A dramatic late fightback by Darlington saw them take all three points.
Peterborough took the lead through James Quinn's first-half goal and only some great saves by debutant Kasper Schmeichel stopped them scoring again.
But the Quakers levelled on 77 minutes - Paul Hopkins slotting home after Posh keeper Mark Tyler had dropped Carlos Logan's corner
And on 88 minutes, Kyle Lafferty - who had earlier had a goal ruled out for offside - rammed home the winner.
Darlington: Schmeichel, Valentine, Clarke, McGurk, Kendrick (Stamp 58), Johnson (Peacock 58), Dickman (Appleby 58), Keltie, Logan, Hopkins, Lafferty.
Subs Not Used: Norton, Webster.
Goals: Hopkins 77, Lafferty 88.
Peterborough: Tyler, Arber, St. Ledger, Plummer, Holden, Kennedy (Farrell 51), Carden, Gain, Newton, Willock (Logan 80), Quinn (Benjamin 79).
Subs Not Used: Huke, Burton.Booked: Newton, Carden, Quinn, St. Ledger.
Goals: Quinn 26.
Ref: J P Robinson (E Yorkshire).