What you need to know:
- De Gea saved Tomas Soucek's shot at the near post in the opening minutes of the second half and minutes later West Ham had the ball in the net again but the referee blew for a foul on De Gea by Michail Antonio.
David De Gea made a huge goalkeeping howler as Manchester United suffered a 1-0 defeat at West Ham on Sunday that threatens to send their season into a tailspin.
Just a few days ago United were heavy favourites to finish in the Premier League's top four and qualify for next season's Champions League but now they are wobbling badly while the Hammers look safe from relegation.
The result hinged on a catastrophic first-half error by De Gea, who let a tame shot from Said Benrahma squirm past him in the 27th minute after his team had largely dominated the opening period.
De Gea's blunder came on the day on which he became the most-capped goalkeeper in Manchester United's history, appearing for the 540th time to move ahead of Alex Stepney.
United, who suffered the pain of conceding a last-gasp penalty in their midweek defeat at Brighton, had been boosted by Arsenal's 2-0 win at third-placed Newcastle earlier in the day.
But although they are still in fourth place, they are now just one point ahead of red-hot Liverpool, although still with a game in hand.
West Ham, who play in the Europa Conference League semi-finals this week, are seven points clear of the drop zone.
David Moyes's Hammers made a bright start in east London, rocking United on their heels before a speedy counter-attack from the visitors, which ended when Marcus Rashford fired over.
With United picking up a head of steam, Bruno Fernandes and then Antony went close.
Rashford hit the outside of the post in the 17th minute afer cutting in from the left as the visitors continued to probe.
De Gea error
But West Ham took the lead against the run of play when Benrahma's weak shot from distance somehow squirmed through the arms of the sprawling De Gea.
The Spaniard appeared to have it covered but his footwork did not look right and despite reaching the ball, his hand was not strong enough to keep the ball out.
He lay prone on the turf while West Ham celebrated, unable to comprehend what had happened.
De Gea has had a mixed season -- he leads the race for the golden glove for most clean sheets in the Premier League but has also made some costly errors.
United hit the post again in the 32nd minute, when the lively Antony's powerful drive was deflected onto the woodwork and behind.
Erik ten Hag's team, who have just 49 goals this season -- the second-lowest tally in the top nine -- managed 11 shots in the first half but only one on target.
As the clock ticked down to half time, De Gea denied Benrahma a second before a huge penalty appeal for handball against Victor Lindelof, which was waved away by referee Peter Bankes.
De Gea saved Tomas Soucek's shot at the near post in the opening minutes of the second half and minutes later West Ham had the ball in the net again but the referee blew for a foul on De Gea by Michail Antonio.
United were rocking badly, looking shaky at the back every time the home side put them under pressure.
Soucek headed the ball into the net with less than 20 minutes to go but the goal was ruled out for offside.
United knew they had to go for broke and Ten Hag threw on Jadon Sancho and Marcel Sabitzer, having earlier introduced Anthony Martial for the ineffective Wout Weghorst.
But although they enjoyed good periods of possession, they looked largely ineffective in attack.
Martial burst through at the start of eight minutes of stoppage time but his shot was palmed away by Lukasz Fabianski and United ran out of time.