It appears that this season is now very likely to end trophyless as some quite astonishingly bad penalties saw United crash out of the cup to Sunderland.
Javier Hernandez had a chance at the back post, but his header was too close to Vito Mannone and the Italian was able to keep it out before his defence cleared well.
United were enjoying all of the early pressure, with Sunderland mostly camped inside their own area, although they did go close when a long ball sat up invitingly for Fabio Borini, with the Italian striking a fierce effort which flew just over the bar.
United again went close as a fine pass from Adnan Januzaj released Shinji Kagawa, who dinked a cross to the back post which Darren Fletcher crashed against the post, but Danny Welbeck was unable to turn the rebound home and Sunderland survived.
But United didn't have long to wait before they were level in the tie. Welbeck flicked on a corner for Jonny Evans at the back post, who took advantage of some woeful Sunderland defending to head past Mannone and put United ahead on away goals.
Sunderland grew into the game more in the second half, however, and began to exert some pressure on the United goal. Adam Johnson had the best chance when the ball dropped for him just twelve yards out, but Buttner made a fine recovery to deny him just as he pulled the trigger.
It was becoming an increasingly nervy finale for United, with Marcos Alonso also going close with a long-range effort before the final whistle heralded the beginning of extra time.
Sunderland were enjoying all the pressure in extra time, although United had a chance to secure their position when Adnan Januzaj showed great skill and strength to lose his market before putting Javier Hernandez miles clear, but the Mexican elected to shoot early with Mannone off his line, and his shot dropped disappointingly wide - a sitter by anyone's standards.
Sunderland continued to attack, but United defended well, looking a completely different team from the game against Chelsea, dealing well with the multiple threats being hurled at them. But just when United were looking like they'd get away with a pretty poor performance, a former player stuck an axe in their back. Ki Seung-Yeung held onto the ball and spun and played Phil Bardsley in for a shot from range, but the Englishman could only blast it straight at David de Gea... who promptly spilled it into the corner of his own net.
It looked like another disaster for Manchester United, who had barely more than a minute left to get a winner. Yet Sunderland immediately retreated, and Adnan Januzaj was able to probe down the left channel, cutting a ball across the corridor or uncertainty for Javier Hernandez to amazingly prod home to send the game to penalties.
Craig Gardner stepped up first, blazing a woeful penalty miles over the bar, before Danny Welbeck repeated the trick, blasting his high and wide. Steven Fletcher then came to take his, but David de Gea made a fine save to deny him. Darren Fletcher was then able to just place his into the bottom corner to give United the lead.
Marcos Alonso scored Sunderland's first afterwards before Adnan Januzaj fired his straight at Vito Mannone to make it 1-1 after three penalties apiece. Ki Seung-Yeung then put his side in front with a cool, composed finish. Phil Jones then stepped up, to sighs of dread within Old Trafford, and fully lived up to them with a dreadful penalty blazed over the bar.
That left Adam Johnson with the chance to win it - his effort was fierce and true, but David de Gea managed to tip it over the bar. United still needed Rafael to score to clinch it, and the fullback looked nervous in his run-up, and though he got it on target, Mannone was able to save to put Sunderland through.