The Eagles join Manchester City, Cardiff City and Liverpool in the last four after goals from Darren Ambrose and Glenn Murray dumped Sir Alex Ferguson's men out of the competition.
Ambrose's stunning strike gave the visitors a surprise lead midway through the first period but Federico Macheda soon equalised from the penatly spot. That sent the game to extra-time, where Murray headed home on 98 minutes to send the Championship side into the semi-finals. United's disappointment, meanwhile, was heightened by first-half injuries to Fabio, Rafael and Dimitar Berbatov (who left Old Trafford wearing a protective boot).
This wasn't a United team full of youngsters, either. Eight of United's starting XI were either current or retired senior internationals. Only Ben Amos, Rafael and Federico Macheda are yet to win a full cap. That meant there was no place from the start for the likes of Paul Pogba, Ravel Morrison, Michael Keane, Zeki Fryers or Larnell Cole, all of whom have featured in this season's earlier rounds.
Crystal Palace, currently 12th in the Championship, enjoyed a bright start in front of their vocal travelling support, with both Wilfried Zaha and Jermaine Easter momentarily sneaking behind the Reds' defence inside the opening 10 minutes.
United weren't overrun, though. The Reds kept possession well, passed the ball on the ground and patiently probed for openings. Darron Gibson, given the task of anchoring the midfield in his first start for seven months, was heavily involved, spreading the ball to both flanks and consistently