Bradford brilliantly responded from two goals down to win 4-2 at Chelsea and dump the Barclays Premier League leaders out of the FA Cup at Stamford Bridge.
League One City, who were 2-0 down after 38 minutes, claimed the biggest scalp of their recent giant-killing exploits to advance to the fifth round and end Chelsea's hopes of challenging for an unprecedented quadruple.
Chelsea reject Filipe Morais levelled the fourth-round tie with 15 minutes to go before Jon Stead, the scorer of City's first, teed up Andy Halliday for the winner eight minutes from time.
And substitute Mark Yeates tucked in a stoppage-time fourth to inflict the Blues' first home loss this season and spark jubilant celebrations from Phil Parkinson's visitors.
Chelsea boss Jose Mourinho said it would be a "disgrace" if his side exited the competition to the League One Bantams, who beat Arsenal and Aston Villa en route to the 2013 League Cup final, where they were beaten by Swansea.
But Mourinho's men were distinctly second best and not even the introduction of Cesc Fabregas, Eden Hazard and Willian off the substitutes' bench could rescue the tie.
It was the first time Chelsea have lost to lower-league opposition at home since 1995 against Millwall.
Chelsea made nine changes with Tuesday's Capital One Cup semi-final second leg against Liverpool in mind, but still fielded a team full of talent worth an estimated £98million. The fees for Bradford's starting XI totalled £7,500.
The Blues must now regroup for their midweek clash with the Reds and next Saturday's Premier League showdown with Manchester City.
Andre Schurrle was conspicuous by his absence from the matchday squad, having been a peripheral figure this term and linked with an imminent exit.
Mourinho on Friday said the World Cup winner would start, but his omission - officially due to a minor muscular injury, and the forward did sit behind the dugout with other non-playing squad members - sparked suggestions the "phenomenal" offer required to seal his departure had been received.
Only Gary Cahill and John Obi Mikel were retained from the starting XI which began the 1-1 draw at Anfield.
Petr Cech played his third successive match at Swansea last week and returned in place of Thibaut Courtois, who has displaced the Czech as first-choice goalkeeper this season and recovered from a thumb problem to start at Liverpool.
Speculation of a move for Cech has dampened of late, but the goalkeeper proved his enduring quality after Gary Liddle's crisp volley was headed over by Cahill.
Morais found Davies from the resulting corner after the defender escaped the attentions of Didier Drogba. His header was powerful, but he was denied by a fine reflex save.
Bradford were threatening at set-pieces, but it was Chelsea who took the lead from a corner when Oscar found Cahill, who flicked the ball in after beating Rory McArdle at the near post.
Drogba, who had required attention on his ankle, forced Ben Williams to save well with an curling shot before Loic Remy and Mohamed Salah, another linked with a move, contrived to fluff another opportunity.
Chelsea could do with the irrepressible energy of Ramires in the closing months of the season and the Brazil midfielder demonstrated his value by winning the ball from Morais at halfway and surging forward.
Ramires, finding fitness after a lingering groin problem, played a one-two with Salah and executed a neat finish to double the lead.
Bradford might have buckled, but former Premier League striker Stead netted with a thunderous shot from the left edge of Chelsea's box.
James Hanson forced Cech to save his header as the visitors enjoyed a spell of pressure on the resumption and Billy Knott, a former Chelsea academy player, curled an effort over and had another blocked by Kurt Zouma.
Salah curled just wide on the break, but Bradford's pressure continued.
Knott and Mikel clashed heads in an aerial duel, with the latter going off to be replaced by Fabregas as Mourinho sent for reinforcements.
Drogba headed a Fabregas free-kick straight at Williams and as Mourinho told Hazard to prepare to come on, Bradford equalised.
James Meredith's long throw was flicked on by Hanson, Knott forced Cech to save and the ball ricocheted to Morais to net into an open goal.
Better was to come when Stead's lay-off was dispatched by Halliday.
Zouma, deployed as an auxiliary striker, shot over from Ramires' cross before Yeates struck a fourth to send the visiting fans delirious.