An injury-time goal by Barnsley's Brian Howard sent Liverpool crashing out of the FA Cup at Anfield and increased the pressure on manager Rafael Benitez.
The Reds led through Dirk Kuyt's first goal since December - but on-loan goalkeeper Luke Steele produced a string of oustanding saves on his debut to keep the Coca-Cola Championship side in the contest.
And, after Stephen Foster headed Barnsley level just before the hour mark, Howard stunned the home side with his late winner.
Liverpool left captain Steven Gerrard on the bench and rested Javier Mascherano, Jose Reina and the almost fit Fernando Torres for this tie, with Tuesday's visit of Inter Milan clearly figuring in Benitez's thoughts.
French goalkeeper Charles Itandje got another run-out, while Jamie Carragher captained the side.
Steele - on loan - was in goal for the visitors, while former Everton midfielder Anderson De Silva was included.
Simon Davey's team had tremendous support at Anfield, with 7,000 Tykes making themselves heard in the Anfield Road end.
In the second minute they were almost celebrating a goal when Istvan Ferenczi's header was blocked by Carragher.
But John Arne Riise and Lucas saw long-range efforts fly wide at the other end, Peter Crouch headed past a post from Steve Finnan's cross, before Kuyt failed to hit the target from Yossi Benayoun's cross.
In the 18th minute Crouch went close with an angled effort that Steele needed to be at full-stretch to touch wide.
Ryan Babel got into the box on the right to fire in a shot that Steele held - but Barnsley had a great chance to opening the scoring in the 28th minute when Daniel Nardiello robbed Sami Hyypia before racing into the box and forcing goalkeeper Charles Itandje into a low save to his right.
Steele was in action again a minute later, clawing away an 18-yard effort from Xabi Alonso - and soon after the Kop let out a roar of relief when they took the lead with Kuyt's first goal since the Champions League win in Marseille in mid-December.
Xabi Alonso swept the ball out to Babel, who made ground into the box before pulling the ball back for the Dutch striker to sweep home from six yards.
Steele did well to keep out a Crouch header on the stroke of half-time - and the home side started the second period strongly with Benayoun and Babel going close.
Barnsley took off Nardiello and sent on Kayode Odejayi, giving Davey's team more of an aerial threat - and in the 56th minute it worked.
Martin Devaney launched an excellent cross on the run from the right, and defender Foster rose to power a header that Itandje got a hand to but could not stop crashing into the net.
Benayoun and Crouch had shots cleared off the line in quick succession, then Hyypia had two efforts blocked as Liverpool stormed forward.
Lucas saw a header turned onto the bar by Steele, who snaffled the rebound - then made another good save after Benayoun's run and shot.
Harry Kewell replaced Babel, before Gerrard was sent on for Lucas after 75 minutes.
The England midfielder's first surging run covered 60 yards and won his team a corner - but at the other end Itandje was looking anything but comfortable.
The Liverpool goalkeeper clattered into Odejayi outside the box, and then palmed away a Howard effort.
Kewell had two efforts blocked, then clipped the bar from 25 yards - but all the noise was coming from the Barnsley fans.
Steele then made a marvellous save to turn over a Kewell hook, and Crouch had the ball taken off his foot in the act of shooting.
The same thing happening to Kuyt seconds after - with Dennis Souza again the defender with the right timing.
Carragher was booked three minutes from time - a caution which will see him banned against Middlesbrough next weekend.
But worse was to follow in the second minute of injury-time when Howard found space on the edge of the box to drill home the winner.
The travelling fans erupted, and they had just 30 seconds to wait for the final whistle and confirmation of a famous victory.