A controversial red card for Cheikhou Kouyate proved costly for West Ham as they were held to a 2-2 draw by Crystal Palace at Upton Park.
The Hammers led 2-1 at the break thanks to Dimitri Payet's stunning free-kick at the end of the half, after Manuel Lanzini had cancelled out Damien Delaney's opening goal.
But Palace grew into the game in the second half and were aided when Kouyate was sent off for a challenge on Dwight Gayle.
That proved decisive as Gayle - who was introduced as a substitute at half-time - scored the equaliser minutes later to earn a valuable point for Palace.
The visitors went ahead in the 14th minute when Bakary Sako's in-swinging free-kick bounced awkwardly in front of Adrian and he palmed the ball straight to Delaney, who headed it into the net.
That lead lasted less than three minutes however as West Ham hit straight back.
Palace failed to clear a cross from the right and the ball fell to Lanzini, who took a touch before confidently rifling the ball into the bottom corner.
Palace had a glorious chance to restore their lead just after the half-hour mark when Angelo Ogbonna misjudged Pape Souare's cross from the left but it caught Jason Puncheon by surprise and he could only divert the ball over the bar with his thigh.
Diafra Sakho came close to giving the Hammers the lead soon after when he outjumped Mile Jedinak at the back post before heading a Payet corner inches wide.
After his heroics for France against Russia in midweek, Payet was largely quiet for much of the opening half but that ended in the 44th minute when he was hacked down by Joel Ward on the edge of the box.
The Frenchman got up to curl an absolutely stunning free-kick right into the top far corner, leaving Wayne Hennessey rooted to the spot.
Kouyate almost cost West Ham a goal in the 66th minute when he under hit a back pass to Adrian and the ball fell to Joe Ledley who teed up Bakary Sako to shoot from the edge of the box but Aaron Cresswell did well to get back and clear off the line.
Things got worse for Kouyate two minutes later when he miscontrolled a pass in midfield before lunging in on Gayle as he nicked the ball away.
Ironically, it was a role reversal from this fixture earlier in the season when Gayle was sent off for a similar offence on Kouyate, with Mark Clattenburg the referee on both occasions.
That red card gave Palace a boost and they almost levelled immediately when Bolasie did brilliantly to beat Cresswell down the right before sending in a perfect cross to the back post, but an unmarked Sako badly miscued his attempted left-foot volley.
The reprieve was short-lived for West Ham however, as Palace got their equaliser minutes later.
Souare provided a consistent attacking threat from left-back throughout the game and he was the source of the goal as he sent in a dangerous cross from the left which Winston Reid missed and Ogbonna only managed to clear as far as Gayle, who calmly passed the ball into the bottom right corner.
Both sides pressed for a winner in the closing stages but neither could engineer one and instead settled for a result that doesn't greatly help either team.
Fifth-placed West Ham lose valuable ground in the race for the top four while Palace remain without a win in the league since December and sit just seven points above the relegation zone in 16th.
Player ratings
West Ham: Adrian (5), Antonio (7), Reid (6), Ogbonna (5), Cresswell (7), Kouyate (5), Noble (6), Emenike (5), Lanzini (7), Payet (7), Sakho (7)
Subs: Carroll (7), Valencia (6), Obiang (6)
Crystal Palace: Hennessey (6), Ward (5), Dann (7), Delaney (7), Souare (7), Jedinak (5), Ledley (6), Zaha (5), Bolasie (7), Puncheon (5), Sako (6)
Subs: Gayle (7), Campbell (5)
Man of the match: Yannick Bolasie