In front of the watching Sir Alex Ferguson, the Reds twice went in front - through Jesse Lingard, in dubious fashion, and Will Keane - only for the Blades to battle back courtesy of Callum McFadzean and Jordan Slew's powerful drives.
A frenetic opening saw United win a free-kick inside the first 25 seconds and two corners before a minute and a half had elapsed. Despite the visitors' enterprising start, the Blades had the first attempt on goal when Corey Gregory headed over a cross from danger-man Slew.
However, United looked the better side and Lingard had three chances inside a minute. The number seven, in the team in place of dead-leg victim Larnell Cole, was almost on the end of a cultured centre from Sean McGinty and then had a shot blocked from a Paul Pogba lay-off. Seconds later, another Pogba cross picked Lingard out and his shot cannoned off George Long and back against the midfielder before looping onto the bar. Harry Maguire appeared to use his hand to keep the bal out but, incredibly, referee Michael Oliver awarded the goal on the erroneous advice of his assistant Gary Beswick.
The Blades would probably have preferred a goal to be given rather than a penalty and a red card for Maguire but the decision was baffling with even the goalscorer bemused.
Maguire made a legitimate block on the line to keep out Ravel Morrison after the playmaker sped through to beat Long following a wonderful burst out of defence by Ryan Tunnicliffe.