Deep into stoppage time a mistake from keeper Steve Simonsen allowed the ball to fall to Kevin Lisbie who finished off the move with a goal.
United had taken the lead six minutes before time, Quinn found Ched Evans on the edge of the box, his miss queued shot fell to Kevin McDonald who fired at goal – his shot was saved by Chris Porter was in the right space to tap home his fifth goal of the season.
That looked to be the winner and came as a huge relief to United who had missed chances all afternoon, but six minutes into stoppage time the home side struck when a free-kick swung in by Dean Cox was miss punched by Simonsen as the ball fell kindly to Kevin Lisbie who drilled home past a crowded box to score.
It was the second time in a week that the Blades had conceded late from a undefended free-kick – Gary Madine taking advantage the previous Sunday to draw the scores level (2-2) in the Steel City Derby.
On-loan Blackpool striker Bill Clarke, who set up the Blades all four goals against Preston in midweek will feel disappointed that he didn’t put away his chance; in the 43rd minute he headed down Quinn’s cross to the turf and agonisingly watched the ball fly over the bar.
Complacency is the key to win matches and may have to be at their best for the visit of MK Dons on Tuesday night.