Graham Potter's return to Brighton & Hove Albion turned into a nightmare when he suffered his first defeat since taking charge of Chelsea in a 4-1 loss in the Premier League on Saturday.
Potter left Brighton to succeed outgoing coach Thomas Tuchel at Chelsea last month, returning to the south coast to face his former employers for the first time complete with a nine-match unbeaten record in all competitions at his new club.
But the hosts made a blistering start at the Amex Stadium against Potter's bewildered side, racing into the lead inside five minutes through a well-taken Leandro Trossard goal.
Breathless Brighton kept coming and soon had a quickfire second after Ruben Loftus-Cheek diverted a corner into his own net, with another own goal from Trevoh Chalobah making it three for Brighton three minutes before the interval.
Chelsea started well after the break and got back into the game three minutes into the second half through Kai Havertz's header, but the damage had already been done and Potter's team could not rouse a comeback.
Pascal Gross's last-gasp fourth goal capped a fine afternoon for Potter's successor Roberto De Zerbi, with the first win of his reign moving Brighton up to seventh in the standings, while Chelsea stay fifth, eight points behind leaders Manchester City.
Determined to make an impression against the man he succeeded in the Brighton managerial hotseat, De Zerbi set his team up to attack from the off.
Veteran Chelsea defender Thiago Silva had already made two goal-line clearances in quick succession to keep out rampant Brighton before Trossard kept his composure to fire his seventh goal of the season into the far corner.
Loftus-Cheek could do nothing as he diverted a corner into his own net, while Pervis Estupinan's low cross proved too hot to handle for Chalobah, sending the home fans into delirium.
Connor Gallagher should have got Chelsea back into the game before Chalobah's unfortunate attempt to clear the low cross went wrong, but the England midfielder made amends when he expertly set up Havertz for Chelsea's goal.
Brighton goalkeeper Robert Sanchez was forced into several fine saves to keep Chelsea from making a contest of it, but it was the home side who sealed a perfect afternoon with Gross's close-range finish on the rebound.
Not only was it the first win of De Zerbi's tenure at the sixth attempt, it was also Brighton's first-ever league win over Chelsea.