Shaun Wright-Phillips dealt a major blow to Chelsea's Premier League title hopes and boosted Queens Park Rangers' chances of survival by giving his side a shock 1-0 win at Stamford Bridge on Wednesday.
Wright-Phillips struck against his former club in the 78th minute to secure his side's second league win of the season and their first away victory since November 2011, as well as their first win at the home of their near neighbours since 1983.
The result took bottom club QPR to within five points of safety, and ensured Chelsea interim manager Rafael Benitez's decision to rest a number of leading players back-fired as his side missed out on the chance to move third.
The Blues' performance will also strengthen their resolve to tie up a deal for Demba Ba.
The Newcastle United striker had travelled to London during the day for talks about a move to Stamford Bridge after Chelsea agreed to trigger the player's £7 million ($11.4 million, 8.6 million euros) release clause.
With Fernando Torres failing to take the one clear opportunity that came his way, this was certainly a game when Benitez would have liked to have been able to call on another striker.
But the manager's team selection left him open to charges of over-confidence against a Rangers side who have endured a grim season and had performed dismally against Liverpool three days previously.
Juan Mata, Ashley Cole, Ramires and Eden Hazard were left on the bench ahead of the weekend FA Cup tie at Southampton, although three of those four were on the pitch at the final whistle, reflecting Chelsea's frustration.
Rangers manager Redknapp also rang the changes, although that was hardly unexpected after his side's meek capitulation against Liverpool.
Redknapp had insisted before the game that his team would not repeat the defensive mistakes that had scarred that performance, and he was good to his word.
It quickly became clear that Chelsea would have to work to find a way through the visiting defence and it took them until the 43rd minute to hit the target, when QPR goalkeeper Julio Cesar saved well with his legs from Oscar's shot.
But by then, Redknapp's side were already convinced they had been the victim of an injustice when Marko Marin - making his Premier League debut - somehow escaped with only a yellow card following an ugly tackle on Stephane Mbia after just four minutes.
Nor was their cause helped by a 15th-minute injury to David Hoilett, who hobbled off with a hamstring problem.
However, that did little to disrupt the visitors, who shut the game down effectively, creating a growing sense of impatience among the home side as the first half drew to a close.
Chelsea had looked short on ideas and energy during the opening period, but they started the second period with more conviction.
Victor Moses was inches away from connecting with Marin's low cross four minutes after the restart, before Branislav Ivanovic headed over from a right-wing corner.
Then Julio Cesar produced an outstanding save to deny Torres from point-blank range.
Rangers were under increasing pressure but they responded well, with Esteban Granero testing Chelsea's stand-in goalkeeper Ross Turnbull for the first time just before the hour.
Home centre-back Gary Cahill was then forced to produce a perfectly timed tackle to deny Jamie Mackie and from the resulting corner, Shaun Derry should have done better than direct his header straight at Turnbull.
The contest had become much more open and Frank Lampard finally had the ball in the net in the 66th minute, only for the effort to be ruled out for offside.
But it was Wright-Phillips who made the breakthrough, drilling the ball home from 20 yards when Chelsea could only half-clear a corner.