MIDDLESBROUGH claimed the first victory of the Garry Monk era with a hard-fought victory over the Blades, who suffered their first league defeat since January 24 - but not without a huge moment controversy.
A Rudy Gestede header on twenty minutes after a Chris Basham error proved the difference, but the hosts were indebted to a massive spot of fortune in stoppage-time when Jack O'Connell's strike from Mark Duffy's free-kick was ruled out, seemingly for offside - with replays showing that no Blades player had strayed offside.
United were the more purposeful early on and fluffed a good chance to take the lead when Leon Clarke delayed in an excellent position after being picked out by Keiron Freeman, with Adam Clayton making a saving block.
Boro regrouped and took the lead, courtesy of a well-placed header from Gestede, who profited from a misplaced clearing header from Chris Basham, with the striker coolly converting with loan keeper Jamal Blackman stranded off his line.
The hosts dominated the rest of the half, with Gestede going close to a second when his header clipped the post following Cyrus Christie's fine centre, while a curler from the impressive Patrick Bamford flashed just wide.
Boro failed to kick on in a scrappy second half, in which the Blades had plenty of the ball, but struggled to create clear-cut chances.
The Blades manufactured one excellent opportunity to level on 63 minutes, with Darren Randolph making a fine save to block Paul Coutts' precision twenty-yarder.
With the game petering out, a major moment of controversy arrived when O'Connell bundled in a leveller at the death, with it being ruled out by the linesman, with replays showing that no Blades player had strayed offside.
In terms of breaching the wins' column, the statistics could not have been more different over the past year of so, with the Blades having triumphed in 31 out of 47 league matches since the beginning of last season, while Boro had picked up a paltry five wins in their previous 43 league games.
The visitors were entitled to be full of confidence and were scenting a club record ninth successive league victory, with one school of thought also suggesting that playing an expensively assembled Boro side who remain a work in progress might not be a bad thing.
With £9m summer signing Martin Braithwaite being a notable absentee on the teamsheet along with centre-half Daniel Ayala, with young defender Dael Fry handed his first home start since February 2016, the Blades' pre-match optimism may have increased further.
The early signs were promising, but Boro's front three of Gestede, Britt Assombalonga and Bamford grew into the game, with the trio providing hope for the future in the final 25 matches of the half.
Bamford, especially, looked the part in the number ten role and linked play well.
The second-half was more attritional and Boro had to show their defensive resolve and organisational and team ethic, but did that well enough, aside from the late scare.
Middlesbrough: Randolph; Christie, Fry, Gibson, Friend; Clayton, Forshaw (Leadbitter 84), Howson; Bamford, Gestede (Baker 68), Assombalonga (Fletcher 80). Substitutes unused: Dimi, Fabio, Roberts, Traore.
Sheffield United: Blackman; Basham, Stearman, O'Connell; Freeman (Brooks 79), Coutts, Fleck, Carruthers (Duffy 57), Stevens; Sharp (Evans 68), Clarke. Substitutes unused: Eastwood, Lundstram, Wright, Lafferty.
Referee: P Bankes (Merseyside).
Attendance: 26,876 (2,142 Sheffield United supporters).