London - After a week of controversy off the pitch, there was an afternoon of rancour on it at Stamford Bridge, as Chelsea's Nemanja Matic was sent off in the Premier League leaders' 1-1 draw against Burnley on Saturday.
Branislav Ivanovic had given Chelsea a first-half lead, but Matic was sent off after the interval and struggling Burnley took advantage to snatch a shock point thanks to Ben Mee's header nine minutes from full-time.
Blues boss Jose Mourinho may feel his side were hard done by as they had three penalty appeals turned down by referee Martin Atkinson, while Matic's dismissal for retaliation was sparked by a nasty tackle from Ashley Barnes.
It was a depressing end to a troubled few days for Chelsea, who could see their lead at the top reduced to five points if Manchester City beat Newcastle later on Saturday.
The club's pre-scheduled "game of equality" came in the aftermath of the shocking incident on the Paris Metro on Tuesday in which a black man was blocked from getting on a train by some of the club's supporters, who were chanting a racist song.
Both captain John Terry and chairman Bruce Buck asserted the club's commitment to anti-racism campaigns in their programme notes and some Chelsea fans also produced banners denouncing racism.
But their attempt to move on was complicated by reports that police are investigating complaints that Chelsea supporters were involved in more racist chanting on their return from the Paris Saint Germain match at St Pancras train station.
For 90 minutes at least, Chelsea could focus on football and Juan Cuadrado offered the first threat with a looping header that Tom Heaton tipped over the bar.
Cuadrado was also instrumental to the home side taking the lead after 14 minutes, even if Ivanovic's goal was all about Eden Hazard's sublime assist.
Having lost the ball near the by-line, Cuadrado did well to win it back, before feeding the Belgian.
Hazard then danced through the Burnley defence, beating two players, and squared for Ivanovic, who finished from close range for his sixth goal of the season.
Chelsea could have been much further ahead by half-time, as Burnley offered virtually no threat of their own, but Atkinson waved away two penalty claims.
For the first, Ivanovic powered a shot at goal, only for it to strike Michael Kightly's hand.
The Burnley midfielder seemed to have his arm stretched out, but Atkinson allowed play to continue. Towards the end of the half, then, Costa surged into the box but was pushed by Michael Keane.
Costa probably didn't help his case by going down easily, although there was a definite shove from the defender.
The second half continued in much the same vein, with Chelsea in total control, and also having more claims for penalties.
Within minutes of the resumption, Filipe Luis burst into the box, only to be felled by Kieran Trippier. Again, Atkinson shook his head.
Chelsea almost made it 2-0 from the resulting corner but Ivanovic couldn't quite get a good enough touch on Costa's header.
Cuadrado was having a hugely productive game and almost scored his first goal.
He slid in for a Hazard cross, only for Mee to just about divert the ball away.
But in the 70th minute Matic was sent off for serious foul play and after the midfielder challenged Barnes, who somewhat dangerously left his foot up, catching Matic on the shin.
The Serbian reacted furiously, confronting Barnes, shoving him to the ground, and Atkinson immediately showed the red card.
It was at that point the game turned, and Burnley scored a shock equaliser in the 81st minute when Mee got above the Chelsea back line to head home a corner, leaving the home crowd to vent their frustration by booing Atkinson.