Yeovil struck a dramatic late winner to move a step closer to their first ever promotion to the second tier of English football.
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Ed Upson nodded home with just six minutes remaining to complete the turnaround and sentence Sheffield United to a third successive season in npower League One.
Gary Johnson's men took just five minutes to wipe out the Blades' 1-0 lead when Upson's top-draw through ball teed up Kevin Dawson.
But after Jamie Murphy's curling effort crashed off the woodwork the Glovers' strike pair of Paddy Madden and James Hayter both missed fantastic chances to put the tie to bed.
Upson's 12th goal for the club teed up a pitch invasion and a clash with Swindon or Brentford at Wembley for a place in the npower Championship.
After a rousing reception it was the unchanged Glovers who made all the early play, Dan Burn heading wide of the back post inside two minutes before the one-goal lead was wiped out.
Yeovil coach Johnson believes that a number of his line-up are already Championship standard and the combination that saw the deadlock broken was certainly would have looked at home in the second tier.
Dawson's well-timed run saw him saw him latch onto a perfectly-weighted through ball from Upson. The Irishman kept his cool before sliding his effort into the bottom corner to nearly lift the roof off Huish Park. Madden may have picked up the League One golden boot before kick off but it was his long-time friend who came up trumps.
Sheffield United, backed by fewer away fans than the Glovers took to Bramall Lane, then had George Long to thank as he denied Joe Edwards' at the near post.
At the other end Murphy came within a lick of point of an equaliser. Callum McFadzean drifted through a posse of green shirts before teeing up the winger who cut inside before bending in a curling effort that thundered off the crossbar.
McAllister, a play-off finalist with Bristol City under Johnson's tutelage back in 2007, saw his drilled effort from a free-kick routine deflected wide after Harry Maguire had hauled back Madden.
Chris Porter, United's hero at this stage of last season's play-offs, fired into the side netting with the first chance of the second half after Burn had done superbly to block his first effort.
Five minutes later Madden missed a guilt-edged chance from close range with the goal gaping after being found by Luke Ayling's delivery.
Maguire's free header with 20 minutes remaining landed well wide as the Blades searched for just their second goal in five games and a return to Wembley for the second-successive season.
Yeovil have been late goal specialists this campaign with 20 scored in the final 15 minutes of games, but with time running out Hayter nodded wide when he rose to meet Foley's left-wing cross.
Midfielder Upson may have only scored one goal from inside the box for the club but he showed Hayter how it is done with just six minutes remaining. The former Ipswich man met Madden's inviting right-wing cross with a lovely glancing header which dropped into the bottom corner to send the Yeovil faithful wild.
With a minute remaining Foley's snapshot was just the wrong side of the post the home side saw out the final minutes to extend Sheffield United's play-off hoodoo to seven attempts and tee up a second Wembley trip in five seasons for the ecstatic Glovers.