In a season of utter misery for United boss David Moyes, it was yet another hammer blow.
His side has seemed to be on the verge of victory thanks to Jonny Evans' first-half goal as the tie ticked towards the final minute of extra time.
But when Phil Bardsley let fly from 20 yards, David de Gea inexplicably allowed the former United man's shot to slip through his grasp.
Amazingly, United earned themselves a reprieve in injury time as his players summoned up the spirit for which they are so famed and with virtually their last attack, Adnan Januzaj crossed for Javier Hernandez to bundle home from close range.
Moyes might have hoped fortune was to smile on him.
Even in a ridiculously bad shoot-out, De Gea's save from Adam Johnson meant Sunderland had only scored twice.
United could not match it though. And whilst it is anticipated Juan Mata will be having a medical tomorrow ahead of a club-record transfer from Chelsea, he will do so against a backdrop of even more gloom.
News of United being close to completing a £40million deal to sign Mata from Chelsea had lifted the sense of foreboding spreading around the club.
With 9,000 Sunderland fans present to roar their team towards the final for the first time since 1985, it made for a lively atmosphere, which might well have triggered Fabio Borini's rash early challenge on Januzaj.
United almost profited from the free-kick, which Javier Hernandez met at the far post, the Mexican's header bouncing down from barely six yards and palmed away by Vito Mannone.
Despite the positive start, United quickly got bogged down, as they have done so often this season.
Neither Wayne Rooney nor Robin van Persie was around to help, having been ruled out through injury, and when Borini launched a dipping 25-yard effort at the United goal, home hearts were in mouths before it dropped just over.
For a good while afterwards, it was all somewhat humdrum. Even Michael Carrick's normally accurate passing was off as the hosts searced for that elusive spark Mata will be expected to supply.
The dynamic changed seven minutes before the interval when, after Darren Fletcher had prodded against a post and Mannone denied Danny Welbeck with the rebound, Januzaj curled over another corner.
Welbeck met it but could not divert the ball on target. However, former United man John O'Shea completely lost his man and as Borini played the Northern Irishman onside, Evans duly finished at the far post.
Having spent two spells on loan with the Black Cats, Evans was biting the hand that used to feed him. In this situation though, that was very much a secondary concern.
Rafael was not too far away from doubling the advantage immediately after the break but Sunderland were putting up a decent showing for their impressively loud fans and a home defence shorn of its suspended skipper Nemanja Vidic was at full stretch to keep the visitors at bay.
Alexander Buttner came to the rescue with a brave block when Adam Johnson swivelled onto Borini's cross and went for goal. Marcos Alonso then sent a volley from the side of the area fizzing past David de Gea's goal.
The loss of Carrick to an ankle injury at the start of extra time robbed United of their most accurate distributor of possession before Hernandez wasted a glorious opportunity, shooting much too early after charging clear and seeing the effort curl well wide.
It was hardly convincing but it seemed United's old scrappers' instinct would see them through.
De Gea's blunder was therefore like a dagger to the heart.
And though Hernandez struck back almost immediately, United's fragile confidence was not strong enough to carry them through a shoot-out where, aside from Darren Fletcher, all the home side's takers missed.
Welbeck and Phil Jones fired over while low efforts from man of the match Januzaj and, decisively, Rafael were saved.
It was shocking stuff.
Sunderland were not much better, with Craig Gardner blazing over and Steven Fletcher and Johnson's efforts saved. But it was enough.
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