Keith Treacy's 80th minute free kick appeared to go in off former United striker Rob Hulse, who ironically had scored the winner for Derby in the same fixture a year ago.
Derby had beaten United that day to celebrate their first win in nearly a year, but it was the visitors who thought they had struck first when Dariius Henderson had the ball in the net.
Jamie Ward had punished hesitancy by Miles Addison in the 19th minute and burst into the area but when his low shot was parried by Stephen Bywater, Henderson had strayed offside before he put the rebound in.
It looked a harsh decision and replays later showed that Henderson was at least level when Ward struck the ball.
Derby should have scored seven minutes later when Paul Green played a ball in behind the United defenders, but Stephen Pearson volleyed over from ten yards.
There had been plenty of hustle and commitment but little quality and the pattern continued in the second half, although Derby did hit the crossbar when a Gary Teale cross deceived Mark Bunn.
The home fans thought Derby had scored in the 76th minute when Pearson flicked on a corner, but Hulse had headed the ball just wide of the back post.
Hulse had got a late winner in last season's fixture, but this time it was the Blades who found a cutting edge to bring the United supporters to their feet.
Treacy swung in a free-kick from the Derby right and although Matthew Kilgallon rose to meet it, it seemed to hit Hulse and loop into the top left corner.
It proved to be the decisive moment although Ward should have added another deep in stoppage time when he was through on goal, but he could not squeeze the ball past Bywater.