Chelsea were plunged into crisis once more as Bournemouth claimed the biggest scalp of their Barclays Premier League campaign with a 1-0 win at Stamford Bridge.
Jose Mourinho thought the reigning champions had turned a corner, but Eddie Howe's visitors inflicted an eighth defeat of the season, the fourth at home.
Glenn Murray headed in moments after coming off the substitutes' bench and, while there may have been a hint of offside, the Cherries deserved their win in front of Chelsea owner Roman Abramovich, who could yet wield the managerial axe.
Bournemouth won the Championship last season and Chelsea the Premier League, with the victory showing not just how far the Cherries had come, but how far the Blues have fallen.
Diego Costa started as a substitute for a second straight game, but, unlike at Tottenham, the striker was not left to brood for the entire 90 minutes and came on at half-time.
Costa's presence was limited until he fouled Adam Smith, was denied a penalty when his cutback hit Simon Francis on the arm and then was shown a yellow card for a foul on Matt Ritchie in quick succession.
Pedro, or Costa, should have scored 15 minutes from time, but neither could get on the end of Branislav Ivanovic's dangerous cross as Chelsea became increasingly frustrated by referee Mike Jones' decisions.
And Mourinho will be furious when he sees the replays of Murray's winner, which came when Thibaut Courtois flapped at a corner and the former Crystal Palace striker headed in.
Chelsea face further examinations against Porto and Leicester in the next 10 days, while the Cherries next play Manchester United.
Eden Hazard received a written report after his Tottenham display as Mourinho wanted to leave no doubt over his delight at the performance.
His first-half appraisal, continuing as Chelsea's "false nine", is unlikely to have been glowing.
The Belgian had a chance to end his search for a first goal of the season in his 20th game in the opening moments, but his shot was less than convincing and straight at Artur Boruc, the goalkeeper back from a six-week injury absence.
Hazard would have been marked down by Mourinho soon after, too, when he lost the ball to Smith, in for the ill Sylvain Distin, and Junior Stanislas shot. Fortunately for Hazard, Gary Cahill blocked.
Courtois was back for the first time in three months after his knee injury and soon showed why he was immediately restored, saving well from Stanislas.
Joshua King could not get the rebound away and then fired at Courtois at the near post.
The visitors attacked with pace and the Blues were vulnerable, particularly out wide.
Costa, who flung his substitutes' bib at Mourinho at the end of last Sunday's draw at White Hart Lane, needed help from John Obi Mikel to put on his green bib after 29 minutes before warming up on the touchline.
Chelsea were ponderous in possession and appeared to lack a presence up front, with Boruc saving from Hazard, Pedro and Oscar.
Mourinho had seen enough, sending on Costa for Oscar.
The muted half-time boos turned to applause just at the sight of the Spain striker going on.
Nemanja Matic, wearing a protective mask, should have done better than head over Willian's cross before Jones ignored Costa's penalty appeal before booking the striker.
Costa stretched to toe-poke Cesc Fabregas' ball over the top, but Boruc saved.
Murray scored eight minutes from time and Bournemouth might have had two more when Ritchie and Harry Arter fired wide on the counter-attack in a memorable win.
TWEET OF THE MATCH
"I think it was very considerate of Chelsea to swap sides with Leicester this season." - Gary Lineker (@GaryLineker) sums up change of fortunes for both Chelsea and Leicester as the champions suffer another Premier League defeat.
Thibaut Courtois: 6 (out of 10)
Branislav Ivanovic: 6
Baba Rahman: 5
Kurt Zouma: 6
Gary Cahill: 7
Cesc Fabregas: 4
Nemanja Matic: 6
Eden Hazard: 5
Diego Costa (Oscar, HT): 5
Bertrand Traore (Rahman, 83): 5
Loic Remy (Fabregas, 83): 5
Artur Boruc: 8
Charlie Daniels: 7
Simon Francis: 7
Steve Cook: 6
Adam Smith: 7
Harry Arter: 7
Dan Gosling: 6
Andrew Surman: 7
Matt Ritchie: 8
Junior Stanislas: 7
Joshua King: 7
Glenn Murray (King, 80): 7
Boruc returned from a six-week absence to frustrate Chelsea, saving from Hazard, Oscar, Pedro and Costa.
MOMENT OF THE MATCH
Murray came off the substitutes' bench to score Bournemouth's winner. There was a hint of offside as he nodded in at the post after Chelsea failed to deal with a corner, with Courtois flapping.
VIEW FROM THE BENCH
Mourinho made one change following the Tottenham draw he regarded as Chelsea's best performance of the season. But Costa was out of the starting XI for a second straight game. Chelsea were again ponderous in possession and lacked a presence, which barely changed when Costa came on at half-time. Bournemouth were without Sylvain Distin due to illness, so Smith came in and goalkeeper Boruc was back. The Cherries huddled round Howe and assistant Jason Tindall at any lengthy break in play and enacted the game plan perfectly.
MOAN OF THE MATCH
Fabregas floundering. One ball to Costa aside, the Spain playmaker again struggled to make a meaningful contribution.
WHO'S UP NEXT
Chelsea v Porto (Champions League, Wednesday, December 9)
Bournemouth v Manchester United (Barclays Premier League, Saturday, December 12)
Mourinho claimed his side were denied a "clear" penalty, but vowed his team would emerge from their difficult spell.
The Portuguese told Sky Sports: "We dominated the game but one refereeing mistake cost us. It was a clear handball, with the result at 0-0, but that's part of the game. In the first half we were not aggressive enough. We had a couple of shots and chances, but in the second half we were much more aggressive.
"We tried in the period where we were stronger but this is the moment when they scored the goal and with five minutes to go with a team that was defending, some people call it intelligent, some people call it lack of play.
"You know, time-outs, stop of the play, asking for a medical department to go in to drink for a minute like in basketball. But they did very well."
Asked if Costa's introduction had made a positive difference, he said: "We were stronger in the second half but it was not because of Diego - it was the attitude, the intensity, the aggressiveness. We started pressing much higher and sooner.
"I'm concerned and I was concerned before this game. You can imagine now after this defeat at home."
He added: "Chelsea will never be fighting for relegation - no chance. Chelsea will win matches and come out of that area. That's not the problem. The problem is that our objective is to finish in the top four."
Howe, in contrast, was understandably delighted.
"We've believed we're on the right path. You look at our recent games and we've done really well, performed well in several of them but haven't got the wins we feel we've deserved," he said.
"Today was a different kind of performance from us. We've had to show a different side to our game but I'm so pleased for the players for all their hard work this season.
"First half, it was quite even. We were well in it. Second half we've played under pressure and we've had to dig in, we've had to defend, put our bodies on the line. I thought we were magnificent defensively. Our keeper played a big part in that. There's been a lot of question marks about our defending this season but we've answered a few questions today.
"I don't think we've defended too badly this season, just individual mistakes, a few setbacks and disappointments have come our way. The players have always come in every day, working hard and try to improve themselves. Every setback they've had, they've bounced back."