Reading super sub Adam Le Fondre struck in the fifth and final minute of injury time to secure a 2-2 draw and inflict yet more misery upon Chelsea manager Rafael Benitez.
The Blues had not performed at their best, but they still looked good value for a vital victory at the Madejski Stadium following goals from Frank Lampard and Juan Mata.
Le Fondre had other ideas, though. The striker scored two late goals to give the Royals a 2-1 win at Newcastle two weeks ago and he rescued Reading again on Wednesday night, first converting Hope Akpan's pass with three minutes left before taking advantage of poor marking to rifle home.
The Blues could have had a penalty when the ball appeared to strike Mikele Leigertwood's arm inside the box, but referee Mark Halsey waved play on.
Gary Cahill wasted a glorious chance to open the scoring when he found space in between four defenders in the box, but his free diving header from Lampard's cross flew wide.
The Reading fans were mocking Fernando Torres - but he stunned them into silence in stoppage time, lifting a delicate chip to compatriot Mata, who lost Alex Pearce and beat Federici from five yards.
Lampard got the goal his performance deserved after 65 minutes when he peeled away from his marker to head Mata's corner in to the far corner.
Brian McDermott acquiesced to the Reading fans' demands for the introduction of Le Fondre, but he took until the 85th minute to make an impact, curling a shot that just cleared Ross Turnbull's goal.
The former Rotherham striker was more clinical with four minutes left, however, latching on to Akpan's pass at the back post before beating Turnbull.
Reading threw everything forward and they got their reward when Akpan nodded the ball to the back post. There Le Fondre, one of three unmarked Reading players, calmly converted, to delight the home fans.