London Irish came from 17-5 down to beat Harlequins 32-17 in the EDF Energy Cup at the Stoop on Saturday.
Four tries in the second half gave London Irish a well deserved victory, with Australian Peter Hewat at the heart of the fightback.
The Exiles scored 27 unanswered points, including three tries in seven second-half minutes, to overturn a 17-5 deficit and snatch a dramatic victory.
Only a month ago, London Irish fought back from 20-3 down to beat Harlequins in the Guinness Premiership.
In his match programme notes, Quins director of rugby Dean Richards promised there would be no repeat on Saturday. He will be cursing his prediction now.
Quins were in charge after a bright opening earned them a 12-0 lead with tries from lock George Robson and winger Charlie Amesbury.
London Irish responded just before the interval through replacement loose forward Richard Thorpe.
Quins re-established their 12-point lead after the re-start with a line-out drive from Neil McMillan - but the Exiles then sparked into gear with quick-fire tries from Peter Hewat, Adam Thompstone and James Bailey.
Hewat then rounded off the victory in the closing minutes after picking off an interception to seal the try-bonus point.
Despite both teams' cup ambitions riding on victory, Richards and Toby Booth both rung the changes.
England's Shane Geraghty made his second start of the season after recovering from injury and there were flashes of his performance that will have pleased the on-looking England backs coach Brian Smith.
But it was Quins' academy winger Seb Stegmann who was heavily involved in the opening exchanges as Harlequins hit London Irish with a high tempo and attacking ambition.
The 19-year-old was halted just short of the line by a brilliant last-ditch challenge from Exiles scrum-half Tom Parker after chasing down Tom Williams' well-weighted chip forward.
London Irish could not clear their lines and Robson drove over for the opening try after six minutes from the back of the line-out.
Stegmann was the catalyst once again as Quins produced some brilliant fluid rugby to create a three-man overlap down the left for Amesbury to score.
Geraghty showed flashes of invention in the first half but the London Irish attacking machine was misfiring.
It took until three minutes before the interval for London Irish to get onto the scoreboard.
Thorpe replaced Jon Fisher and virtually his first action was to attack from the base of a London Irish five-metre scrum and he drove over for the try.
Quins threatened to seal the game at the start of the second period.
Parker needed to pull off another try-saving tackle after hooker Chris Brooker burst out of a driving maul - but the respite was brief as Quins executed a classic catch and drive for captain McMillan to score.
It was almost as if that score sparked London Irish into action - and replacement scrum-half Alfredo Lalanne, who only arrived in the country this week, was at the heart of it as the Exiles scored three tries in seven minutes.
Lalanne brought pace and momentum to the Exiles game and Hewat took full advantage, picking a magnificent line to glide through the Quins defence for a brilliant solo try.
Thompstone then took the opportunity to haunt Harlequins, who released him last season, by stepping off the right wing to beat two defenders and draw the scores level at 17-17.
London Irish had their tails up and winger Bailey raced over for his first try since signing from Gloucester last month.
Facing another Exiles fightback, Harlequins sent on the big guns in the form of England scum-half Danny Care and fly-half Chris Malone.
But Quins then suffered another set-back when replacement hooker Ignacio Elosu was sin-binned after the touch judge spotted his boot had connected with Thorpe's head at a ruck.
Quins came close after a midfield break from Care but Quins could not engineer a comeback and Hewat's interception try sealed the bonus point and rounded off another Exiles fightback.
Tries: Robson, Amesbury, McMillan
For London Irish:
Tries: Thorpe, Hewat 2, Thompstone, Bailey
Cons: Hewat, Hickey
Harlequins: 15 Tom Williams, 14 Seb Stegmann, 13 Tosh Masson, 12 Epi Taione, 11 Charlie Amesbury, 10 Waisea Luveniyali, 9 Gareth Williams, 8 Tom Guest, 7 Neil McMillan, 6 Phil Davies, 5 George Robson, 4 James Percival, 3 Mark Lambert, 2 Chris Brooker, 1 Aston Croall.
Replacements: 16 Ignacio Elosu, 17 Alex Rogers, 18 Jim Evans, 19 Chris York, 20 Danny Care, 21 De Wet Barry, 22 Seb Jewell.
London Irish: 15 Peter Hewat, 14 James Bailey, 13 Dominic Shabbo, 12 Charlie Gower, 11 Adam Thompstone, 10 Shane Geraghty, 9 Tom Parker, 8 Jonathon Fisher, 7 Steffon Armitage, 6 Declan Danaher, 5 James Hudson, 4 Gary Johnson, 3 Faan Rautenbach, 2 James Buckland, 1 Clarke Dermody (c).
Replacements: 16 Dan Murphy, 17 David Paice, 18 Bob Casey, 19 Richard Thorpe, 20 Tom Homer, 21 Alfredo Lalanne, 22 Eoghan Hickey.
Referee: Greg Garner (England)
Touch judges: Roger Baileff (England), David Kurk (England)
Assessor: Tony Spreadbury (England)
Television match official: Bob Ockenden (England)