Chelsea were a goal ahead when Thiago Silva was given his marching orders, which saw their lead unravel as goals from Mattheus Pereira, Callum Robinson and Mbaye Diagne sealed West Brom's comeback.
Chelsea saw their afternoon implode on them on Saturday as they let a one-goal lead slip to lose 5-2 to relegation battlers West Brom.
But their day swiftly turned sour when Thiago Silva was sent off just minutes later for a second yellow card.
West Brom piled on the pressure and played some good football going forward, though ultimately it was Chelsea's errors which let the visitors back into it.
Matheus Pereira was the beneficiary on two counts in first-half injury time, breaching Chelsea's back line and lobbing Edouard Mendy to equalise before adding a clever finish put the Baggies ahead.
Thomas Tuchel threw on Mason Mount in the second half to try and galvanise his troops, but Callum Robinson soon struck the killer blow, finding the net with a thumping volley after a beautifully crafted team move.
Mbaye Diagne then added a similarly worked goal soon after to the scoresheet, before Mount's consolation offered a brief notion of a comeback, until Robinson's late goal sealed West Brom's important victory.
Here are the talking points from Stamford Bridge.
1. Pulisic pounces but doesn't take his chance
He's been used sparingly since Thomas Tuchel's appointment, but Christian Pulisic was given an opportunity to stake a claim for a regular spot in the XI.
The American impressed on FA Cup duty prior to the international break and was the Blues' main threat early on against West Brom.
He won the free-kick from which Chelsea's opener came, and while they had a stroke of fortune when Alonso's effort cannoned back off the post, Pulisic reacted quickly to follow up superbly and tuck the ball home.
The 22-year-old dropped off between the lines looking for the ball but was unable to get Chelsea going in quite the same way as Mason Mount, who then replaced him at the break.
Pulisic may well have scored, but Tuchel had clearly seen enough, which suggests he still has some work to do.
2. Silva sees red
For all his experience Thiago Silva made the afternoon a lot more difficult for his side with a red card after just 29 minutes.
Fortunately for the Brazilian his side had only just taken the lead moments earlier, but it still put Chelsea on the back foot.
In truth, Silva may well have been fortunate not to have been sent off earlier when he pulled down Pereira.
As it was he was dismissed for a second yellow, perhaps somewhat harshly, but his tackle was still a tad unruly.
Jorginho's poor pass had put Chelsea in danger but Silva overcooked his desperate lunge to stop the shot and took out Okay Yokuşlu right in front of the referee.
Maybe David Coote had thought twice about his earlier decision, but either way, Silva could have no complaints.
3. Hospital defending
It was Big Sam football at its finest, although in stark contrast to what West Brom had produced for the rest of the half.
The Baggies looked to be heading into half-time a goal down despite pegging Chelsea back for much of the opening 45 minutes.
But with just 10 men the hosts began to wobble, and were soon beaten by a simple route one hoof downfield from West Brom keeper Sam Johnstone.
It was a decent kick, and Matheus Pereira's run and lob over Edouard Mendy was sublime; but what were Chelsea's defence doing? Stood like statues mainly. And it wasn't the only time they were caught napping.
West Brom pressed Chelsea, who were adamant on playing out from the back, and it cost them twice.
Jorginho's misplaced pass had already put them in trouble in the build-up to Silva's red card. Then Reece James sent a difficult pass to Andreas Christensen, allowing West Brom to break and Pereira to score his second.
Take nothing away from Pereira for the finish, but Chelsea were their own worst enemies.
4. Blues' top four charge takes hit
This was not what Thomas Tuchel signed up for.
All that pre-international break progress had put Chelsea in good stead to achieve a top four finish this season.
One defeat against West Brom won't change that, but it will certainly raise a few alarm bells with Chelsea's manager.
They may have taken the lead but Chelsea were abject from the first whistle and struggled to penetrate West Brom until their goal against the run of play.
The game did hinge on Silva's red, but too many players were going through the motions, and even those that weren't - i.e Reece James - then made simple errors which proved costly.
And once they were behind West Brom carved Chelsea open so easily.
Tuchel's men have as good a chance as anyone for a place in the Champions League next season, but not playing like this.
5. Baggies run riot
It was only in September that the Baggies were three goals up against Chelsea and capitulated to draw 3-3.
Pereira was clearly keen to reverse that trend, and Chelsea played straight into his hands.
The Brazilian lifted a delightful effort over Mendy for the first, before fooling several of Chelsea's players with his disguised shot into the corner for his second.Chelsea rallied in the second half to try and recover but the damage by then, was too great, as Pereira had laid bare their defensive frailties and could and probably should have completed his hat-trick.
Instead he left it to his teammates, Robinson and Diagne, to apply the final nails, as West Brom played the ball around effortlessly and counter attacked with venom. They were excellent.
At the same time, they have been given a huge lifeline in their battle against the drop. Surely Big Sam can't escape it again?
