Benjamin Sesko came off the bench to deny West Ham a vital win in their hunt for Premier League survival as his 96th-minute goal earned a 1-1 draw for Manchester United at the London Stadium.
While West Ham cost the visitors a fifth straight win under Michael Carrick, Tomas Soucek's tap-in wasn't enough to move them level on points with 17th-place Nottingham Forest after Sesko's second injury-time goal in three substitute appearances.
On a night that offered a different test for Carrick's side, Manchester United had few answers for a rejuvenated West Ham, who allowed the away side the majority of possession.
An inspired Aaron Wan-Bissaka denied his former side the opener in the first half when he cleared Luke Shaw's effort off the line after a well-worked corner routine.
The first half was no comparison to the second, with Jarrod Bowen bullying Shaw to set up Soucek's goal in front of England boss Thomas Tuchel five minutes after the break.
Casemiro headed home in the 62nd minute but the Brazil international's goal was ruled offside following a VAR check, which was the narrowest of calls.
West Ham's dropped points offers Forest the opportunity to pull clear of the Hammers as they take on Wolves on Wednesday. Manchester United stay fourth.
West Ham boss Nuno Espirito Santo believed his side should have secured three points given their late opportunities but commended the Hammers' overall display.
"[I'm] disappointed, of course, all of us [are]," Nuno said.
"The players, the fans, and ourselves are disappointed to take this final goal in the final moments of the game.
"We competed really well. We've been able to contain a very good team that was in a good dynamic. You can see the quality, the effort of the players, the organisation.
"The way we started the second half was really positive. We went, we scored. We defended really well. It was a good chance. But overall, a very positive performance on the defensive aspect of the game.
"Also, in the final moments, we had two or three situations with a good space in behind that. We should have put the game to bed."
Man Utd boss Michael Carrick said his side continuing to show their fight-back-ability is a positive sign.
"Mixed feelings," he said. "We know we weren't quite at our best, and credit West Ham for that, closing certain spaces up, but we can be better.
"The boys were a bit frustrated and disappointed, with that, which is a really good sign for me. And then finishing with a late goal when we've had to find one is a good moment, and another positive for us to know that we can do that.
"And we don't want to have to pull on it and use it too often but in the grand scheme, there's a point, something we can take, and we kind of dust ourselves down a little bit, assess it, take these games off, a five-game period, and to just have one draw in there is a big positive."
