Grant McCann's injury-time rocket completed a superb comeback for Posh.
Goals from Theo Robinson either side of the half-hour mark put Derby in control. Robinson then blew the opportunity of a hat-trick and the chance to put the game beyond Peterborough when he skied an effort over the crossbar.
It was to prove costly as Paul Taylor pulled a goal back before Tommy Rowe equalised early in the second half and McCann stole the points for Posh.
Injury and suspension left Peterborough manager Ferguson with just 16 players to choose from, prompting him to include 15-year-old striker Jannai Gordon Hutton among the substitutes.
The only change for Posh from the defeat to league leaders Southampton in midweek saw Emile Sinclair come in for Lee Frecklington, who was sent off at St Mary's.
An Injury list in double figures also limited the options of Derby boss Nigel Clough, who included his own 15-year-old striker on the bench in the shape of Mason Bennett.
Clough made four changes after the heavy home defeat to Cardiff on Wednesday night. Jake Buxton and Mark O'Brien came into the defence while Robinson and Conor Doyle were recalled up front.
Robinson threatened early on and Taylor replied soon afterwards but neither player tested the opposing goalkeeper.
Derby's Frank Fielding was the first to be worked when he tipped a 20-yard effort from George Boyd over the crossbar after 22 minutes.
Derby took the lead in the 28th minute. Both Jamie Ward and Robinson broke the offside trap after a long ball over the top from O'Brien but it was the latter who latched on to the pass and beat Paul Jones in the Peterborough goal.
Four minutes later it was 2-0. Ward got free down the left-hand side and his cross just eluded Gabby Zakuani, enabling Robinson to steer the ball home from just inside the area.
Robinson could have all but sealed the win and completed his hat-trick but he fired over the crossbar.
Taylor made him pay when he collected Mark Little's pass and fired into the roof of the net from 15 yards eight minutes before half-time.
Peterborough were level after 51 minutes. It looked a lost cause as Boyd chased down a Derby defender, who attempted to shepherd the ball out of play. However, Boyd managed to poke the ball into the danger area and Rowe slotted home from close range.
Jones ensured that all Peterborough's hard work did not count for nothing when he tipped a shot from Ward round the post just after the hour.
There were half-chances at either end as both sides went for the victory, which came Peterborough's way when McCann blasted home a left-footed volley from the edge of the area in the third of five added minutes.
