An injury-time winner knocked the Blades out of the Capital One Cup at the first round stage.
Chris Hussey, who also got Burton Albion's opener, ensured David Weir suffered a first defeat as Blades manager.
Michael Doyle netted with an impressive strike but the Blades lost to Burton for a second consecutive season.
Boss Weir made four changes for the Capital One Cup first round tie. Flynn, Coady and McFadzean all started, as did goalkeeper Howard, who, a year earlier, made his Blades debut against the Brewers in the same competition - a game that ended in a penalty shoot-out.
United's first two chances of the night both fell to McFadzean - his first flew over the crossbar and the second, after a string of passes, was deflected wide after Porter neatly laid off. Lone striker Porter was the next to go close, on 16 minutes, he headed towards goal from a corner but goalkeeper Pickford, who recovered from a mix-up with Sharps, tipped around a post.
Burton's first opportunity was the result of Hussey's run down the left and his deep cross reached Bell at the back post but the subsequent shot was comfortable for Howard.
The Blades, however, were clearly on top and hit back with a couple of chances before the half-time break. Following Coady's run, Porter's shot forced a save, low to his left, from Pickford, and then Westlake cut in from his right flank but curled over.
Burton, though, began the second half well and opened the scoring with their first serious assault on the United goal. Howard could only direct McGurk's cross to HUSSEY, who blasted into the corner of the net with 50 minutes on the referee's watch.
Shortly before being replaced by Taylor, Porter twice went close in the same move after a neat and patient build-up. McDonald picked out the striker, who saw first shot saved by Pickford and the follow up poked wide, although he had little time to adjust. Flynn's shot was also close on 58 minutes, after cutting in from the left, his shot appeared to be destined for the top corner before it deflected off the head of Holness.
Flynn, however, was decisive in United levelling the scores on 64 minutes. He was fouled and then played a quick free-kick inside for the unmarked DOYLE to line up a 25-yard shot that dipped over the head of the Burton goalkeeper and into the top corner.
Substitute Taylor had three efforts in quick succession, firing into the sidenetting, the goalkeeper's arms and then wide, and he was then joined by Brandy, who replaced Flynn, with little over 15 minutes to go.
In the final ten minutes both McDonald and Taylor went over in the penalty area but referee Ilderton was unmoved by either incident, whilst the visitors won a couple of corners in the five minutes of added time, plus a shot from Bell was over the top, before they struck the winner, deep into injury-time.
Substitute Macdonald delivered a deep cross and HUSSEY again proved to be a thorn for the Blades, planting a powerful header past Howard. There was time for a Taylor effort, but he could not direct McDonald's cross into the net and Burton hung on for a second consecutive League Cup win over United.
Blades: Howard, Westlake, Williams, Maguire, Flynn (Brandy 74), Doyle, Porter (Taylor 61), McDonald, Collins, Coady, McFadzean.
Unused: Hill, McGinty, Murphy, Long, Whitehouse.
Burton: Pickford, Edwards, Hussey, Delap (Kee 79), Sharps, Bell, Weir (Palmer 72), McCrory, McGurk, Holness, Symes (Macdonald 79).
Unused: Reed, Lyness, Knowles, Dyer.