Jesse Lingard, who spent half of last season on loan at the Hammers, predictably scored the game's winning goal after coming off the bench in the second half.
The Englishman's strike to make it 2-1 to United in the 89th-minute was an absolute peach as well, but the goal was far from the end of the drama at the London Stadium.
In the final minute of stoppage-time, West Ham were awarded a late penalty after Luke Shaw was found guilty of handling the ball inside the area when attempting to block a cross.
David Moyes then decided to make the situation even more dramatic by bringing Mark Noble off the bench to take the spot kick.
The West Ham captain's first touch of the ball was his effort from 12 yards and in a cruel twist of fate, Noble's weak shot was saved by David de Gea.
West Ham United vs Manchester United Reaction | Football Terrace
It's without question one of the craziest endings to a Premier League fixture and one Hammers fan filmed himself while it was all unfolding in front of his eyes.
The West Ham supporter known as 'Stevo The Mad Man' was full of confidence when he saw Noble being brought off the bench and even boldly said: "He doesn't miss."
Take a look at the video posted to his official Twitter account here...
Video: West Ham fan's reaction to Noble penalty miss
That reaction to the penalty miss is absolutely priceless.
To be fair, most West Ham fans would have been full of confidence when they saw Noble striding up to take the penalty, despite the fact he'd only just entered the fray.
The Englishman's record from 12 yards is stellar and prior to the defeat versus United, he hadn't missed a spot kick since 2016.
But that impressive run came to an end on Sunday and in hindsight, Moyes should have probably let Declan Rice take the penalty.