Daniel Sturridge rescued Chelsea from FA Cup humiliation but Andre Villas-Boas was let down by his players again as Birmingham secured a 1-1 draw in the fifth round at Stamford Bridge.
The home side's season was in danger of hitting a new low when their latest defensive calamity allowed David Murphy to give the visitors a shock half-time lead at Stamford Bridge.
Colin Doyle also brilliantly saved a Juan Mata penalty and although Sturridge eventually equalized, Chelsea simply did not do enough to find a winner.
Frank Lampard, Michael Essien and Jose Bosingwa all missed out as Chelsea made four changes for today's FA Cup fifth-round tie against Birmingham.
Ramires, John Obi Mikel and Gary Cahill were recalled after injury and illness, Ryan Bertrand was handed a rare start at left-back for the injured Ashley Cole, while Didier Drogba and Salomon Kalou were surprise substitutes having returned from the Africa Cup of Nations.
Championship promotion chasers Birmingham made five changes, drafting in Colin Doyle, Pablo Ibanez, Morgaro Gomis, Nathan Redmond and Adam Rooney.
Defeat on Saturday was unthinkable for a Chelsea side who had not won since their fourth-round victory at QPR, slumping to fifth in the Barclays Premier League.
That had piled the pressure on manager Andre Villas-Boas, who admitted his "project" no longer enjoyed the support of all of his players.
But the Portuguese insisted he retained the full backing of billionaire owner Roman Abramovich, while his position was strengthened yesterday when Guus Hiddink became manager of Anzhi Makhachkala.
That did not stop the traveling fans singing, "You're getting sacked in the morning", as Chelsea toiled.
They were almost embarrassed when David Luiz's attempted clearance deflected off a Birmingham head before Stephen Carr was forced off injured, replaced by Jonathan Spector.
Villas-Boas had branded last weekend's defeat at Everton the "worst performance of the season" but this was no better and the calamitous defending that cost them at Goodison returned when they fell behind in the 20th minute.
David Luiz and Raul Meireles failed to deal with a near-post corner and the ball ran on to David Murphy, who drilled home.
Chelsea finally woke up and urgent attacking immediately won them a penalty when Wade Elliott tripped Ramires - but Colin Doyle produced an excellent save to tip Juan Mata's spot-kick onto the post.
Curtis Davies produced a brilliant interception to stop Fernando Torres reaching a cross from Daniel Sturridge, who then screamed in vain for another penalty when he crumpled under Ibanez's challenge.
Luiz tried to inject some inspiration when a typical surge forward saw Jordon Mutch rightly booked for the crudest of challenges, which the defender almost punished with a superb free-kick that Doyle tipped over.
Sturridge then unleashed a blistering volley that just dipped too late and Mata directed a header too close to Doyle before Chelsea trudged off to muted jeers.
The boos turned to cheers after the interval when Torres, who had another half to forget, was hauled off and replaced by Drogba.
The Ivorian immediately showed the kind of commitment lacking from Torres by retrieving the ball for a goal-kick.
But Chelsea showed little sign of improvement and Cahill was booked for a tangle with Rooney before more shocking set-piece defending saw Redmond blast over a corner.
Kalou replaced the ineffective Mikel, although it was someone else the crowd wanted, judging by the chants of "Super Frankie Lampard".
However, the change paid off, Mata drilling wide after brilliantly taking down Luiz's crossfield pass before Sturridge equalized in the 62nd minute.
Branislav Ivanovic was given an age to deliver a cross and Sturridge rose unmarked to power home a header via the inside of the post.
Elliott threw himself in front of Ivanovic's attempted follow-up before Rooney was withdrawn for Jake Jervis.
Meireles' low drive through a crowd of players was deflected just over the bar while Jordan Mutch tested Petr Cech with a low free-kick.
Lampard finally joined the fray when he replaced Mata for the last seven minutes in another unpopular substitution, with Elliott also withdrawn for Chris Burke.
Ivanovic appealed half-heartedly for a penalty after a coming together with Murphy before Birmingham wasted a glorious chance to snatch it when Redmond shot tamely at Cech after racing clear.
Lampard drilled a free-kick straight at the wall in stoppage-time, Luiz was cautioned for clattering into Spector, and Davies headed too close to Cech as the visitors held on for a deserved replay.
