Semi Ajayi headed a late equaliser to stun Premier League champions Liverpool and clinch a 1-1 draw for relegation battlers West Brom at Anfield.
Liverpool took an early lead when Sadio Mane finished clinically from a Joel Matip pass (12) but they struggled to create clear chances after that and Sam Allardyce's side punished their sluggish display with a deserved leveller eight minutes from time.
Matheus Pereira was the provider, his cross from the right finding Ajayi, whose towering header crept over the line after bouncing off the inside of the post.
West Brom had come close to equalising before that, when Alisson Becker pulled off a one-on-one save to deny Karlan Grant, and Liverpool's woes were compounded by an apparent groin injury to Matip, who had to be replaced by Rhys Williams after an hour.
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The draw means Liverpool missed the chance to move five points clear at the top of the Premier League table and leaves Allardyce, the last manager to win a Premier League game at Anfield, with Crystal Palace in 2017, to celebrate another impressive result on Merseyside.
How West Brom denied Liverpool
Liverpool, 7-0 winners over Crystal Palace at Selhurst Park in their last outing, started with the same sort of attacking intent back at Anfield, forcing West Brom backwards right from the start of the game.
Liverpool: Alisson (7), Alexander-Arnold (6), Matip (7), Fabinho (6), Robertson (7), Henderson (6), Wijnaldum (6), Jones (6), Salah (5), Mane (7), Firmino (5).
Subs: Williams (5), Oxlade-Chamberlain (6), Origi (6).
West Brom: Johnstone (7), Furlong (8), Ajayi (8), O’Shea (8), Gibbs (7), Phillips (7), Sawyers (7), Gallagher (6), Diangana (6), Robinson (7), Grant (6).
Subs: Pereira (7), Austin (6), Ivanovic (6)
Man of the match: Darnell Furlong
Andrew Roberton's over-lapping runs were the main source of danger, but Mane's opener came from an unlikely provider in Matip.
The centre-back's diagonal pass was powerfully struck, but Mane's chested control took him away from Ajayi and left him with only Sam Johnstone to beat.
His volleyed finish was ruthlessly dispatched but for all their dominance of the ball - they finished the game with 78 per cent share of possession - Liverpool lacked cutting edge from then on.
Dangerous crosses flew into the box from both flanks as they pushed for a second goal before the break, but West Brom defended resolutely, clearing their lines repeatedly and absorbing wave after wave of Liverpool pressure.
- Mohamed Salah returned for Liverpool while Curtis Jones replaced Naby Keita in midfield..
- West Brom's only change saw Callum Robinson included in place of the suspended Jake Livermore.
Mohamed Salah fired narrowly over while Mane was denied a second goal by a superb piece of last-ditch defending from Darnell Furlong, and Liverpool's frustration continued after the break.
Jordan Henderson bent a shot narrowly wide with the first chance of the second period, but West Brom, sensing an opportunity to take something from the game, soon began playing with some attacking intent of their own.
Their belief grew when Matip, Liverpool's only fit senior centre-back, was forced off with a groin injury, and Jurgen Klopp's frustration was evident when he was shown a yellow card after reacting angrily to a decision to award a foul against Matip's replacement, Williams.
The first real warning sign for Liverpool came in the 69th minute, when Grady Diangana screwed a shot wide from the edge of the hosts' box when he should have done better, and Alisson's one-on-one save from Grant came just a few minutes later.
Trent Alexander-Arnold bent a free-kick narrowly wide from 30 yards up at the other end as the 2,000 fans inside Anfield attempted to rouse the hosts, but West Brom were immediately on the attack again and Ajayi's leveller, headed in off the inside of the post, was well deserved.
Liverpool tried to force their way back in front in the closing stages, but Johnstone denied Roberto Firmino with an outstanding save in the final minute of normal time allowing the Baggies to cling on for a deserved point.
For 19th-placed West Brom, it is a result which raises hopes of survival under the newly-appointed Allardyce. For Liverpool, however, it is a lesson in the dangers of complacency.
Klopp: Dropped points our fault
Jurgen Klopp told Sky Sports: "It was our own fault. It's an incredible challenge to stay active, to stay lively, when you are constantly facing 10 men but to be fair West Brom did that. They did their job for 90 minutes.
"West Brom deserved the point tonight because we didn't finish the game and didn't have enough clear-cut chances.
"It is difficult to create - it's a busy period and all that stuff.
"But in the end everyone knows and feels we should have and could have done better tonight and then we would have won the game."
Allardyce hails 'fantastic' Baggies
Sam Allardyce told Sky Sports: "I saw really good organisation and a team trying their very best to nullify the best team in the league on their own patch.
"What a fantastic job they did. It's all about staying in the game when you come here and not opening up too much and not allowing Liverpool to breeze through you winning three or four-nil.
"We kept that tight ship going for a long period of time, which I thought was very good considering we went a goal down so early on in the game."
Opta stats: Big Sam's Anfield run continues
- West Brom manager Sam Allardyce is unbeaten in each of his last four Premier League games at Anfield (W1 D3), each coming while in charge at a different club (Sunderland, Crystal Palace, Everton, West Brom).
- Liverpool have failed to win six of their 15 Premier League games this season (W9 D5 L1), as many as they did in the entirety of last season in the league (W32 D3 L3).
- Having picked up just one point from their opening six away league games this season (D1 L5), West Brom have since earned two from their last two on the road away to Manchester City and Liverpool (D2).
- Liverpool failed to win for only the second time in their last 34 home league games (W32 D2), also drawing 1-1 with Burnley in July of last season.
- Sadio Mané scored his 69th Premier League goal for Liverpool, moving him level with Luis Suárez and behind only Robbie Fowler (128), Steven Gerrard (120), Michael Owen (118) and Mohamed Salah (86) for most goals in the competition for the Reds.
- Among defenders since the start of the 2018-19 season, only Virgil van Dijk and Aaron Pierre (both nine) have scored more headed league goals in England's top four tiers than West Brom's Semi Ajayi (eight).
Liverpool travel to Newcastle on Wednesday, with kick-off at 8pm. West Brom host Leeds on Tuesday, kicking off at 6pm.