Sheffield - Scotland's Oliver McBurnie fired Sheffield United into fifth place in the Premier League as a calamitous mistake and more VAR drama condemned West Ham to a 1-0 defeat at Bramall Lane on Friday.
McBurnie clinched the points in the second half when the striker took advantage of a blunder from Hammers keeper David Martin, whose errant pass was missed by the equally culpable Fabian Balbuena.
West Ham's Robert Snodgrass thought he had equalised in stoppage-time, but the strike was controversially ruled out on review after VAR spotted a handball by team-mate Declan Rice before he provided the assist.
The Blades had gone three league games without a win after successive defeats at Liverpool and Manchester City.
But Chris Wilder's side, enjoying a remarkably successful first season back in the top-flight, are now just four points behind Chelsea in fourth.
Wilder tipped David Moyes to ensure West Ham avoid relegation following his arrival to replace the sacked Manuel Pellegrini.
But this was a frustrating evening for Moyes, who was beaten for the first time after winning the opening two matches of his second spell with the club.
Moyes has a big task on his hands with West Ham sitting 16th, just two points above the relegation zone.
Earlier on Friday, Wilder signed a contract extension that will keep him at his boyhood club until 2024 and he was given a rapturous reception by Blades fans before kick-off.
The upbeat atmosphere nearly got even more boisterous when Oliver Norwood swung in the corner and John Egan headed just over.
West Ham suffered a blow when goalkeeper Lukasz Fabianski appeared to injure his hip while taking a goal-kick in the 14th minute.
Fabianski, who looked distraught as he hobbled off, had only just returned from a three-month absence with a hip problem.
Martin had deputised for some of Fabianski's time on the sidelines and he was called on to step in again, with disastrous results.
Felipe Anderson should have lifted West Ham when the Brazilian seized on Egan's misdirected header and ran clear on goal, but he shot narrowly wide of the far post.
West Ham were making their first visit to Bramall Lane since 2007 and many of their fans turned up in Carlos Tevez masks in reference to the Argentine striker's crucial and controversial role in helping their side avoid relegation at the Blades' expense 13 years ago.
Those travelling Hammers supporters are hoping to avoid another relegation dogfight, but they won't have been encouraged by their team's second half display.
The Blades nearly took the lead immediately after the interval when John Fleck's shot smacked into Pablo Zabaleta and the loose ball rebounded to the midfielder, who should have done better than drill wide with an open goal at his mercy.
But Wilder's team only had to wait until the 54th minute to open the scoring.
Martin tried to play a quick goal-kick short to Balbuena, but the wayward pass escaped the dozing West Ham defender and Fleck pounced, squaring the ball for McBurnie to drill under the former Millwall keeper for his fourth goal of the season.
Martin had wept tears of joy when he kept a clean-sheet on his West Ham debut in their win at Chelsea earlier this season, but there was only pain etched on his face this time.
Blades keeper Dean Henderson made a fine save from Manuel Lanzini's free-kick before denying the Italian again with another good stop in a tense finale.
And when Snodgrass beat Henderson at the near-post in the final seconds, West Ham's celebrations were curtailed in agonising fashion as VAR intervened to chalk it off for Rice's handball.