David de Gea gifted West Ham a vital three points in their fight for Premier League survival - and plunged Manchester United's Champions League hopes in jeopardy.
After an all-round abject display - summed up by De Gea's poor error in the 27th minute, allowing Said Benrahma's strike to squirm in - United are just one point ahead of arch-rivals Liverpool in the top-four race after their narrow win against Brentford.
West Ham, meanwhile, moved up to 15th with the deserved 1-0 win - even if the only goal of the game was rather fortuitous. Driven by Declan Rice in midfield, David Moyes' men are close to securing their place in the top flight next season ahead of their Europa Conference League semi-final clash with Dutch side AZ Alkmaar on Thursday.
Here are six talking points from the London Stadium.
1. Ten Hag sees the warning signs
Game after game this season, Manchester United fans have been forced to brace themselves for at least one wayward kick from De Gea which leads a dangerous opposition attack. This time it took just five minutes, with the Spaniard dallying with the ball at his feet before gifting Declan Rice a pass 30 yards from goal - just liked he'd do to Michail Antonio at the start of the second half.
His early error afforded the West Ham captain the perfect opportunity to drive forward in an open start to the match which also saw United squander a series of early chances through Marcus Rashford, Bruno Fernandes, Antony and Christian Eriksen. Thankfully for De Gea, Wout Weghorst was there to chase back and produce a superb sliding tackle in the box to block Rice's cross.
Still, Erik ten Hag was rightly furious and gave his goalkeeper a dressing down from the touchline after yet another lapse in his unreliable distribution. Although De Gea remains in line for a new contract on reduced terms, talk of a new 'keeper coming in this summer isn't going away and it's easy to see why. The 32-year-old is a liability and Ten Hag knows it.
2. De Gea's latest low
If he's not good on the ball, at least he's good at keeping it out of his net - right? That's long been the defence of De Gea, who leads the Premier League Golden Glove race for the most individual clean sheets.
But now there's another, less prestigious chart which he ranks top of thanks to a howler to gift West Ham an opener in the 27th minute. After Benrahma broke away down the middle of the pitch and was allowed room to shoot by three United defenders, the Algerian winger produced a tame shot which looked like a routine save for De Gea.
Spoiler: it wasn't. The ball trickled towards goal and somehow went through his right wrist, squirming into the back of the net as the London Stadium watched in astonishment.
De Gea, with his head on the turf and Ten Hag even angrier, had made his fourth error directly leading to a goal this season, more than any other top-flight player. Perhaps that new, reduced-terms contract won't be sanctioned by the United board after all - especially after some more powderpuff goalkeeping in the second half, most notably when Antonio got the better of him at a corner but ref Bankes blew for a foul.
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3. Lindelof's lucky escape
If the contingent of glum United supporters - who've had a mammoth few weeks of travelling - thought things couldn't get any worse to cap a ponderous first-half performance from their team, Victor Lindelof almost added to their misery. Attempting to block Benrahma's cross in the penalty area, the Swede awkwardly stuck his arm out and the ball hit his hand in line with his rib cage.
The exact area of the contact was evidently enough for VAR Stuart Attwell not to advise Peter Bankes to take an on-field review, but pundits and fans alike were baffled after watching replays of their own at half-time, with Gary Lineker tweeting: "Clearly a handball by Lindelof. Remarkable that VAR didn't intervene."
4. Weghorst's woeful return
After more than a month without a start, Ten Hag decided to give Weghorst another go from the off in the hope that he'd come back stronger. 56 mostly unimpressive minutes later and Ten Hag had seen enough, hooking the Netherlands international - once again played as No.10 - in what surely is a sign that he won't be staying with United beyond his loan spell.
Anthony Martial came on in his place, and although he'd do well to take a leaf out of Weghorst's book in terms of work rate, it's clear that the French striker is the better option in the long term. Martial's introduction also prompted a change of shape, allowing Rashford to move out to the left and Fernandes inside, a much more natural set-up for the trio.
5. Light at the end of the tunnel for Moyes
With a European semi-final on the horizon, it's hard to believe that things have been so miserable for West Ham this season. That's because, after back-to-back top seven finishes, Hammers fans have endured a relegation battle, with Moyes struggling to get a tune out of his players on a consistent basis.
Now, though, West Ham are playing their best football of the season - not exactly a great feat - and ended a run of three successive defeats with his emphatic win to put them 15th in the table and seven points clear of the drop zone. Dogged defending coupled with cutting counter attacks should've led to more goals, but Benrahma took the glory with the only goal.
It's AZ Alkmaar up next in the Europa Conference League before an opportunity to reach the magic 40-point mark at Brentford next Sunday.
6. United's away-day misery continues
Having lost 1-0 at Brighton in the 99th minute on Thursday to ensure that they've failed to win any of their eight games visiting the current top nine in the Premier League this season, the Red Devils sank at another away ground on Sunday to continue their utterly awful record on the road. The game will be defined by De Gea's error, but his team-mates didn't do much to help.
Ten Hag has made Old Trafford a fortress this term, losing just two games in all competitions, but stinging defeats at Anfield, the Etihad, the Emirates, St James' Park and in Seville all scream of a team lacking the mental fortitude to perform when the going gets tough. Lucky enough for United, they've only got Bournemouth left to visit. Mind you, a draw might be a good result in context.
