Edwin van der Sar blew the Premiership title race wide open with two errors that gifted Portsmouth a 2-1 win over Manchester United on Saturday.
Van der Sar was to blame for both of Portsmouth's goals while United found home goalkeeper David James at his inspired best as they slumped to a first league defeat since 21 January.
Matt Taylor pounced on the first blunder before the Dutch ‘keeper missed Rio Ferdinand's backpass to concede the second. John O'Shea got one back for United in the last minute but Portsmouth held on.
Alex Ferguson's side are now just three points ahead of Chelsea, who won 1-0 against Tottenham Hotspur earlier on in the day, and the meeting between the clubs at Stamford Bridge on 9 May looks likely to decide an increasingly dramatic title race.
Cristiano Ronaldo had United's first sight of goal in the 11th minute when he put a 25-yard free kick wide of David James's right hand post.
Ronaldo had the Pompey defence on the back foot again two minutes later when he raced past challenges from Stefanovic and Linvoy Primus before shooting into the side-netting from the edge of the penalty area.
Primus produced a heroic sliding tackle to deny Wayne Rooney a shooting charge on the quarter hour mark after Ronaldo has turned O'Shea's long clearance into his path.
Richardson should have done better in the 24th minute when he was picked out by a Ronaldo free-kick and blasted his left-footed shot high over the Portsmouth crossbar and into the stands.
Portsmouth stunned the travelling United fans when they took the lead 15 minutes before half-time, with Van der Sar at fault for the second time in four days.
Van der Sar was blamed for Roma's second goal in Wednesday's 2-1 defeat in Italy when he punched a free-kick straight into the path of an opposition forward.
The Dutchman repeated that error when he only parried Benjani's long-range shot, allowing Taylor to slide in and smash the rebound into the back of his net.
Michael Carrick should have equalised straight from the re-start when he latched onto Paul Scholes's defence-splitting past only to shoot wide from close range with only James to beat.
James came to Pompey's rescue a few minutes later when he saved with his legs from point blank range to deny Fletcher an equaliser after Carrick had opened up the defence.
Van der Sar made amends for his error in first half stoppage time when he came racing out of his goal to block Taylor's chipped effort after Wes Brown had failed to deal with O'Neil's long ball over the top.
Ferguson bought on Ryan Giggs at the start of the second half in place of the hapless Kieran Richardson.
Van der Sar was almost caught out again ten minutes after the re-start when he punched Nwankwo Kanu's 20-yard pile-driver straight back into the danger zone but this time there was nobody following up to take advantage.
Darren Fletcher was replaced by Ole Gunnar Solskjaer in the 58th minute and the Norwegian could have scored with his first touch.
James pulled of a breathtaking one-handed save to keep out Ronaldo's volley from a Giggs corner and Solskjaer's follow up was cleared from the line by Lauren.
James needed to be at his brilliant best to turn Giggs' curling long-range shot over the bar soon after.
Van der Sar matched James's heroics with a brilliant flying stop to turn Sean Davis's 65th minute volley around a post and deny the midfielder his first goal of the season.
The Dutch international was at fault for Pompey's crucial second goal when he needlessly came racing out of his goal and allowed Ferdinand's back pass to trickle past him for an own goal.
O'Shea gave United hope when he tapped home a stoppage time consolation strike after James had spilled Solskjaer's shot but it wasn't enough to spare Ferguson's side.
