West Ham's nemesis Glenn Murray scored a controversial late equaliser as Brighton snatched a 3-3 draw at the London Stadium.
The veteran striker, starting a Premier League match for the first time since September, grabbed his seventh career goal against the Hammers in the 79th minute.
The ball appeared to hit Murray's arm before he fired home but a VAR check decided otherwise, to disbelief in the stands when the replay was shown.
It completed a remarkable comeback for Graham Potter's side, who were 3-1 down thanks to an Issa Diop effort and a double from Robert Snodgrass.
West Ham defender Angelo Ogbonna had already scored an own goal before Pascal Gross pounced on some woeful defending to halve the deficit and Murray found the net to secure a share of the spoils.
A draw meant West Ham slipped into the relegation zone, and with trips to Manchester City and Liverpool next up, it might be a while before they climb back out.
Brighton boss Potter was on the touchline despite the death of his father just two days earlier.
His side should have taken the lead after just 90 seconds but Aaron Mooy put a free header wide.
West Ham handed a debut to new signing Tomas Soucek and the Czech midfielder almost grabbed a debut goal but his header, from a Snodgrass free-kick, was saved by Mathew Ryan.
Leandro Trossard and Murray squandered further chances before, on the half-hour mark, another Snodgrass set-piece found the toe of the sliding Diop to give West Ham the lead.
It was two on the stroke of half-time after Martin Montoya's headed clearance fell to Snodgrass, who hit a volley into the ground and off the shin of Adam Webster to leave Ryan with no chance.
Brighton needed a slice of luck to get back into the game, and they received it moments after the restart.
Lukasz Fabianski felt he was being impeded as he attempted to clear a Gross corner, but he succeeded only in punching the ball onto the shoulder of Ogbonna and into his own net.
But Snodgrass restored the two-goal cushion when he took aim from the edge of the box and scored with the aid of another deflection, this time off the head of Bernardo.
West Ham should have been home and hosed but contrived to let Brighton in again when Diop failed to notice Gross lurking behind him.
The German whipped the ball away and prodded it past Fabianski and just inside the far post.
Then came Murray's moment from Davy Propper's cross, and a lengthy VAR check which decided the ball had not brushed his arm before he tucked it away.
Brighton could have stolen all three points at the end when Solly March's free-kick flew towards the top corner but Fabianski clawed it away to at least preserve a point for the hosts.
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