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.
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.
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.
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.