Posh boss Barry Fry blasted his side after a spineless second half show cost them dearly as Chesterfield came back to secure a 2-1 win at Saltergate.
Leading by a Richard Logan goal and in total control, Peterborough looked on course for their fifth away win of the season.
Chesterfield left the field at half-time to a chorus of boos but an inspired team talk from boss Roy McFarland turned the game around.
Marvin Robinson equalised within minutes of the restart and striking partner Glynn Hurst grabbed the winner with 15 minutes left.
Afterwards Fry conferssed: "Im baffled, completely baffled. To say I was upset with the second half performance is an understatement.
"We could of had three or four inside the first 30 minutes, but their keeper pulled off some great saves and he deserves to be man of the match.
"But after half-time we were a disgrace and, even when we had chances, we failed in front of goal." Meanwhile Roy McFarland was elated after a victory which gives his side renewed hope of avoiding the drop.
"In the first half we looked as though we were still sitting round the Christmas dinner table rather than playing a game of football.
But, after the break, we were magnificent and even if we had lost I'd of thanked the lads for their great attitude.
Fry opted to play three up front and his gamble paid off after just 12 minutes when a defensive error gave Steve Thomson a shooting chance.
Keeper Carl Muggleton got a hand to the shot only to steer the ball into the path of Logan who sided footed into an empty net.
Chesterfield did not have a shot until the half hour and were lucky not to fall further behind.
After the break, Robinson headed home at the far post on 48 minutes and Hurst wrapped up a deserved win when he robbed Mark Arber on the edge of the box before beating James Pullen with a low drive.