The Stags were 3-0 up inside 17 minutes in North Lincolnshire, with Keiran Murtagh, Louis Briscoe and Lee Stevenson all finding the target.
But a hat-trick by Hakeem Adelakun, a controversial spot-kick by Matt Godden and a late strike from Curtis Bateson, consigned us to a shock defeat.
Our reserves went ahead after just five minutes when Briscoe’s right-sided corner was met superbly met by Murtagh, who crashed a half-volley into the net.
Briscoe then doubled our advantage when he poked home from a tight angle, after Lindon Meikle’s clever through ball put him in the clear.
Our side were celebrating again on 17 minutes when Ross Dyer’s knockdown fell kindly for Stevenson, who rifled a 25-yard strike beyond a full-stretch James Severn.
Dyer and Jake Speight both came close to extending our advantage, before the hosts reduced arrears on 34 minutes, thanks to Hakeeb Adelakun’s well-placed finish.
The Stags could have made it 4-1 before the interval, however, as both Speight and Meikle both had attempts parried away by Severn in stoppage-time.
Our side also created two opportunities in quick succession after the re-start as Speight fired straight at Severn, before Dyer headed wide from Black’s cross.
With less than 20 minutes to play, our team still held a two-goal cushion, but The Iron showed steel as they launched a stirring comeback.
Adelakun made it 3-2 with a bullet left-foot strike, before substitute Matt Gooden was judged to have been fouled by Liam Marsden inside the area.
The decision to award a penalty by the referee was very harsh on The Stags, but Godden slammed the resulting spot-kick beyond Ian Deakin, to draw Scunny level.
Both teams pressed forward in search of a winner in the closing exchanges, but with seven minutes remaining, The Iron completed a remarkable turnaround.
Adelakun completed his hat-trick as he cut inside from the left flank and unleashed a powerful right-foot strike, which Deakin was unable to tip over the woodwork.
The Stags were shell-shocked and in added time, Scunny compounded our misery as Curtis Bateson raced through to make it 5-3.
Team: Deakin, Marsden, McCombe, Birch, Black, Briscoe, Stevenson, Murtagh, Meikle, Dyer, Speight.