It was the first of only two occasions Liverpool have been beaten in the last 51 games, a loss the signalled the end of a seven-month unbeaten run.
And barely had the final whistle been blown at the London Stadium when Michail Antonio was cheerfully sharing just how West Ham United had achieved such an increasingly rare feat.
"The whole plan was to stick it in straight on the keeper and we've just challenged," said the forward.
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"We saw that they struggle with the ball coming in on them and all we had to do was keep the ball away from Virgil (van Dijk). It's worked out for us.
"The plan was to push straight on the keeper and no-one marked me on the keeper so I'm there to block him and make sure he can't come out and catch it. You then saw what happened: goals."
No bones were made about the Hammers' decision for Antonio to jostle and harass Alisson Becker which led to the Liverpool goalkeeper punching the ball into his own net after just four minutes, setting the tone for an unsettled display from the Brazilian back in November.
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Alisson could have done better when attempting to keep out Pablo Fornals' strike for West Ham's second goal while the ultimate winner from Kurt Zouma saw Liverpool undone again at a set-piece as they tumbled 3-2.
That was the fourth time in six Premier League games Liverpool had leaked more than once, having previously drawn 2-2 at home to Manchester City and Brighton after being held 3-3 at Brentford.
Lessons, though, have clearly been learned by Jurgen Klopp's side.
In 15 subsequent top-flight matches, the Reds have conceded more than one goal in a game only twice, in the creditable 2-2 draws at Tottenham Hotspur and Chelsea.
Indeed, those four goals contribute to a total of only nine leaked during that period, compared to 11 in the first 11 Premier League matches of the campaign.
Liverpool's prowess at set-pieces has been the subject of much intrigue, not least due to the impact of the neuro11 initiative, the latest profit of which came with Takumi Minamino's second goal in the FA Cup win over Norwich City on Wednesday.
While no team has scored more times this season in the Premier League from dead-ball situations, only three clubs - Chelsea, Wolverhampton Wanderers and Manchester City, the three stingiest defences in the division - have conceded fewer goals from such threats.
And since the West Ham defeat, Liverpool haven't leaked once from a set-piece in the Premier League, given Mateo Kovacic's wonderstrike when volleying in from 20 yards after Caoimhin Kelleher had punched out a free-kick at Chelsea in January doesn't fall within the traditional parameters.
Saturday evening's visit of the Hammers, though, represents the biggest threat yet with only the top three having been more deadly at set-pieces this campaign.
Alisson, having taken a backseat to Kelleher at Wembley last weekend, underlined his position as first choice between the sticks with an impressive stop in midweek to prevent Norwich substitute Jonathan Rowe snatching a late equaliser.
And the keeper will know exactly what to expect from David Moyes's side. How the Liverpool outfield players protect Alisson at corners, then, could be key to avoiding a repeat of events at the London Stadium.
Plus, as Antonio also readily admitted back in November, the likely reunion of Joel Matip and Virgil van Dijk at centre-back will make life difficult for the Hammers.
"They're two very strong centre-halves, who are always up for a fight," he said. "It's probably one of the hardest games I've had this season, so it's a good thing I only face them one more time this season."
That time is now. And Liverpool have no excuse not to know what could be coming.
