Kaoru Mitoma's second-half winner sent Brighton through to the FA Cup fifth round at Chelsea's expense, with the comeback victory easing the slight pressure on Seagulls manager Fabian Hurzeler.
The Seagulls were in the firing line after their dismal 7-0 loss to Nottingham Forest last week and started this cup tie in horrific fashion - with goalkeeper Bart Verbruggen fumbling Cole Palmer's harmless cross into his own net after five minutes.
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But Chelsea have had their issues recently and one of them is letting leads slip. They were pegged back seven minutes after that early Verbruggen gift as Georginio Rutter nodded Joel Veltman's delicious cross past former Seagulls goalkeeper Robert Sanchez.
Chelsea could have re-taken the lead before half-time but Christopher Nkunku - starting up front with Nicolas Jackson and Marc Guiu both injured - chose to pass to Palmer instead of shoot when through on goal. The England international could only put his headed effort onto the roof of the net.
Brighton took their chance after the break, as Mitoma was played in by a delicate Rutter pass - the finish even more sumptuous over the flailing Sanchez.
It was a cup tie which exposed Chelsea's carelessness but also lack of cutting edge up top. Since Boxing Day, they have taken the lead then failed to win the game on five occasions.
They also ended this game with one shot on target - it came after four minutes when Palmer stung Verbruggen's gloves from distance.
Brighton, meanwhile, move into Monday's fifth-round draw ending their mini-slump of back-to-back Premier League defeats - though Chelsea can get revenge next week, as these two are reunited on Valentine's Day, live on Sky Sports' Friday Night Football.
Friday 14th February 7:00pm Kick off 8:00pm
More to follow.
Analysis: Chelsea's nightmare after Christmas continues
When Chelsea last met Brighton in late September, they scored four goals. Cole Palmer got them all. Crucially, they were in the run of being the Premier League's great entertainers.
Now the Blues' output appears to have frozen and stuck. This was the fifth time since Christmas where Chelsea have taken a lead and not seen the win through. Leads have been chucked away for dropped points against Fulham, Crystal Palace, Bournemouth, Man City and now Brighton.
After their last defeat against City, Gary Neville accused Chelsea of missing an opportunity at the Etihad and their FA Cup exit followed the same story.
They faced a Brighton side who shipped seven last weekend, saw their goalkeeper chuck one into his net after five minutes on Saturday and even captain Lewis Dunk came off at half-time to destabilise the Seagulls' backline even further. But Chelsea could muster just one shot on target - which came just four minutes in.
After scoring four in the last encounter between these two, Palmer was nullified. Christopher Nkunku's rustiness was clear to see - his 33 touches was the second-lowest tally out of Chelsea's starters. He also failed to have a shot all game.
Nicolas Jackson could come back for the next trip to Brighton on Valentine's Day but even he has gone eight games without a goal. How Chelsea get out of this rut of not scoring enough and throwing away leads is unclear at the moment.