Manchester United suffered a humiliating Carabao Cup exit as they were beaten in a marathon penalty shootout by League Two Grimsby at Blundell Park.
It had appeared United had rescued themselves in the nick of time after finding themselves two goals down in a truly awful first-half display, with returning goalkeeper Andre Onana at fault for Grimsby's second.
Substitute Bryan Mbeumo pulled one back before Harry Maguire equalised in the final minute.
But Matheus Cunha failed to secure victory in United's last spot-kick of the regulation five, and Mbeumo's second penalty hit the bar to hand Grimsby a 12-11 shootout success and trigger a pitch invasion from the jubilant home fans.
United manager Ruben Amorim gave the impression of cowering in the dugout as the drama unfolded and one of only two realistic chances of silverware disappeared at the first available opportunity.
He endured the 'sacked in the morning' chants from gleeful home fans sat in the stand behind him during the first half as he watched his side find themselves in an even more embarrassing position than those he experienced last season, during which he described them as 'maybe the worst' in the club's history.
He was not helped by Onana's shocking error. Left out for the first two Premier League games of the season, the Cameroon international had not looked entirely comfortable even before he came to collect a cross swept into a crowded penalty area after 30 minutes.
Inexplicably, Onana dropped it and former United academy player Tyrell Warren turned the loose ball into an empty net.
The goal doubled the advantage given to Grimsby by Charles Vernam, who beat Onana with a low shot after Kieran Green took possession away from Amad Diallo and Manuel Ugarte, allowing Darragh Burns to deliver the far-post cross.
Amorim sent on skipper Bruno Fernandes, new forward Mbeumo and defensive kingpin Matthijs de Ligt at half-time in an attempt to save the game.
Mbeumo pulled one back with a shot into the corner 15 minutes from time. Following more attacking substitutes that meant Mason Mount and Mbeumo were filling the wing-back roles by the end, Maguire converted the last-minute equaliser.
As Cunha strode up to take United's fifth penalty, it seemed they would escape. But Cunha failed – and so did his team.