Christy Pym the penalties hero as Mansfield Town stun Peterborough United to reach Carabao Cup round four
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It also preserved Stags' unbeaten record in all competition this season.
A pulsating first half saw Mansfield go ahead through a Will Swan spot kick.
However, Clarke-Harris beat him again just after the break.
But Stags grew into the game in the last half-hour, suddenly dominating. And they got their just rewards as Lucas Akins netted a stoppage time spot kick to send the game into penalties for the second round in a row.
Clough made five changes with George Williams, Baily Cargill, Hiram Boateng, George Maris and James Gale replacing Jordan Bowery, Calum Macdonald, Ollie Clarke, Davis Keillor-Dunn and Lucas Akins.
A very strong-looking Posh, who sit eighth in the League One table, went into the game unchanged from the weekend and clearly keen on progressing in the competition.
Peterborough started on the front foot and almost grabbed a third minute lead.
Jones got down the left and no one cut out his cross, which reached Clarke-Harris, but his finish was cleared off the line by Cargill.
Instead it was the Stags who swept ahead on five minutes.
Swan was fouled by Collins in the box and, after Collins was booked, Swan stepped up to confidently fire home the spot kick down the centre.
Posh quickly began to turn up the pressure again and Burrows lashed a shot across goal and beyond the far post on eight minutes.
Two minutes later Swan and Lewis set up Maris to fire just wide at the other end as the thrilling start continued.
Mason-Clark was too high with a rising shot on the turn from 18 yards in Posh's next raid.
Pym had his first big save to make on 18 minutes as Kyprianou lofted a ball over Flint and Mason-Clark ran onto it and fired low on target, the Stags keeper with a firm wrist to turn it away diving full to his left.
Three minutes later Lewis did fantastically well to chase and retrieve a long Pym kick against the odds by the corner flag and lay it back to Williams, whose cross was headed wide by Gale from 12 yards. The Stags striker stayed down and needed treatment before he could carry on.
Pym safely held the ball at his near post as Jones tried to get the ball on target from an impossible angle on the left by-line.
Reed was booked for tripping Mason-Clark on 29 minutes to concede a free kick centrally from 25 yards.
Clarke-Harris smashed the kick down the centre and it went through the wall and past a rooted Pym to level the game on the half-hour.
Peterborough were almost level a minute later as Pym tipped over a flicked header by Kyprianou.
Stags then had to soak up more sustained pressure as Posh dominated the ball, but they kept a good shape and saw it out.
Taking the heat off themselves, Stags then kept possession in their own half as the sides went in all square.
Bowery replaced Reed for Mansfield for the second half, playing on the right.
But within little over a minute Posh were in front. Williams cleared Kioso's low cross from the right almost off the line but Clarke-Harris was quickly there to sweep home the rebound.
Clarke-Harris almost completed a hat-trick on 55 minutes, but his low shot was wide of the far post.
Stags made a triple change on the hour with Akins, Macdonald and Clarke replacing Cargill, Maris and Swan.
Clarke Harris was wide with a thunderous free kick while Stags escaped from another corner as the ball pin-balled wide off two players.
After a half-cleared first home corner on 66 minutes Lewis cleverly guided the ball into the path of Bowery 20 yards out and he forced Bilokapic into his first save of the contest.
Burrow was booked for a foul on 68 minutes and from that Gale saw his far post shot deflect into the sidenetting.
A minute later Gale somehow agonisingly headed over on the line when Boateng's deflected shot looked like it might be heading straight in.
Rejuvenated Stags were pushing hard now and thought they had levelled on 71 minutes as Akins turned home Gale's cross but an offside flag quickly halted celebrations.
With the keeper out of position and the goal gaping Macdonald was wide from just over the halfway line with an ambitious punt forward.
Boateng then volleyed horribly over from a Clarke cross as Posh defended stoutly, then Clare flashed a shot wide from 20 yards.
Mason-Clark and Flint were booked for clashing on 87 minutes and, from the free kick, Gale headed over at the far post.
Six minutes were added in which Flint headed a Lewis cross at the keeper and Gale had a volley deflect over.
And on 93 minutes persistent Stags were finally rewarded as they won a penalty when Macdonald was brought down by Clarke-Harris.
Akins sent Bilokapic the wrong way and the home fans exploded with relief and delight as went to a shoot out.
Akins safely netted Mansfield's first before Pym turned away Clarke-Harris' effort and Stags had the advantage.
Bowery made it 2-1 just inside the left post and Pym then again performed heroics to save from Edwards.
Flint's blast then hit the left post to give Posh hope and Mason-Clark made it 2-1 before Clarke's kick was saved.
But Knight missed the chance to level as his kick hit the outside of the post and Keillor-Dunn sent Bilokapic the wrong way to secure a famous win.
STAGS: Pym, Williams (Keillor-Dunn 74), Flint, Brunt, Cargill (Macdonald 60), Reed (Bowery HT), Lewis, Boateng, Maris (Clarke 60), Gale, Swan (Akins 60). SUBS NOT USED: Flinders, Cooper, Flanagan, Abdullah.
POSH: Bilokapic, Burrows (Sturge 89), Edwards, Knight, Clarke-Harris, Mason-Clark, Poku (Katongo 89), Jones (Ajiboye 74), Kyprianou, Collins, Kioso. SUBS NOT USED: Talley, Crichlow, De Havilland, Randall, Fernandez, Corbett.REFEREE: Sebastian Stockbridge.
ATTENDANCE: 4,372 (682 away).