Steve Carson capped a terrific Barnsley fightback as his first senior goal condemned Peterborough to another night of home misery.
Midfielder Carson dashed into the box to convert a Kevin Betsy cross on 63 minutes as the visitors quickly turned a 2-1 half-time deficit into a matchwinning lead.
Barnsley had trailed to a Leon McKenzie double strike which sandwiched a Betsy effort at the break, but afterwards they took control.
First Carson created Isaiah Rankin's leveller two minutes into the second period before tucking away the decider.
Boss Gudjon Thordarson said: "We were unusually soft in the first half and I didn't particularly like what I was seeing, but in the end we came from behind twice to get a very handy three points.
"I always thought we looked dangerous when we were going forward and felt it was only a matter of time before their defence buckled.
"I'm delighted for Steve to create one goal and score the other to make it a great night for us." McKenzie's opener which arrived after good work by strike partner Andy Clarke on 28 minutes and a deft second three minutes from the interval after a surging Calum Willock run.
But defensive sloppiness allowed Rankin to reply with a deflected 20-yard drive before Betsy burst past his man on the Tykes right and laid on the winner for Carson.
Dejected Posh manager-owner Barry Fry said: "It is becoming a nightmare.
"We keep saying 'we don't deserve this' and 'we don't deserve that' but when you defend in the way we did it is no surprise we haven't won at home.
"And I honestly don't know how to go about sorting the problem out."