Crystal Palace inflicted serious damage on Chelsea's Premier League title hopes by defeating their fellow Londoners 1-0 at home, thanks to a John Terry own goal.
There was little goalmouth action early on before the Blues had a clear opening to go in front when Cesar Azpilicueta crossed for Andre Schurrle, who couldn't hit the target from close range.
Palace then finished the half as the stronger side and probably should have had a penalty just after the half-hour mark.
Yannick Bolasie appeared to be brought down by Gary Cahill, but referee Jon Moss thought that the England international made contact with the ball.
Soon after half time, Terry had a great chance to put the visitors ahead but couldn't hit the target when a loose ball from a corner dropped his way.
Terry did find the net moments later but it was at the wrong end. The Blues' skipper misjudged Joel Ward's cross and inadvertently flicked past Petr Cech to put the home side in front.
Chelsea came close to an instant equaliser when Eden Hazard curled for the far corner and was denied by Julian Speroni's outstretched hand.
Terry then almost made amends for his own goal by heading Oscar's corner against the top of the crossbar.
With just 17 minutes left, Hazard looked like he had equalised but Speroni against denied the Belgian, before Palace had a pair of great chances to put the game to bed.
First, Cameron Jerome struck the foot of the post with a shot from inside the area, and then Joe Ledley fizzed just wide.
Palace continued to threaten on the counter-attack as Chelsea struggled to carve out a late opening to equalise.
It's now two away defeats in a row for Chelsea, who remain top of the table but could be overtaken by both Manchester City and Liverpool.
Palace, meanwhile, are now five points clear of the bottom three.