It was a catastrophic moment. Not only did it contribute to Tottenham blowing that game, the eventual 3-2 defeat was part of a two-match losing streak that ultimately cost them a top-four place.
If that makes it sound like it was Walker's momentary lapse in judgement alone that led to what followed, well, it did not.
Liverpool were already showing signs of coming back into that match. Walker's error just compounded an all-round frustrating afternoon's work from Spurs.
He did not even play in the 1-0 loss to Fulham that followed a week later.
It was, though, probably the most painful moment of a season that proved to be the toughest of Walker's fledgling career.
While not for a second underestimating the effort that has propelled the 23-year-old to this point, prior to 2012-13, he had been on a largely upward trajectory.
The Sheffield United youth product was snapped up by Tottenham (along with fellow Blade Kyle Naughton) in 2009 having barely played a dozen senior matches. As part of the deal, Walker was loaned back to the Yorkshire outfit in order to gain more experience.
On the bigger stage of the Premier League, Walker's exciting forays forward made many take notice. This included then Spurs manager, Harry Redknapp, who installed him in his side for the beginning of the following campaign.
Walker would start all but one Premier League fixture, making 48 appearances in all competitions. Swiftly settling into the Spurs defense at right-back, his performances saw him voted PFA Young Player of the Year.
Having featured in 50 games this time around (the most of any Spurs player this season), there was hardly a dramatic drop-off from Walker in 2012-13—but he did not have it as easy.
After a year and a half spent in the top flight, opposition players had visibly cottoned on to some of his attacking tendencies. No longer an unknown quantity, he would have to work harder to make a contribution in the final third.
Well aware of the speed Walker possesses, teams granted him less time and space, both on and off the ball. They could not stop him getting forward, but they could restrict the route choices available to him in the final third.
Aaron Lennon went through something similar after his first season at Spurs. However, like his similarly quick teammate, Walker worked even harder to make his contributions count (directly setting up a further three goals than the year before).
Walker also had to deal with changes of personnel around him. He adjusted nicely enough to different right midfielders playing in front of him—his remit in getting forward had hardly changed.
Where he proved less adaptable was in compensating for the changes next to him, in defense.
Having played alongside Younes Kaboul (with Ledley King also usually besides him marshaling them) for much of the previous season, Walker was now usually next to either William Gallas or Michael Dawson. On certain occasions, he did not even play, with Naughton preferred.
Andre Villas-Boas' frequent rotation of the defense did not aid Walker's progress. It became clear he was a player who benefited from the consistency of knowing the tricks and traits of his fellow defenders.
Walker's play improved after Dawson was restored to the team in favor of the erratic Gallas. Even then, the generally unsettled nature of the defense was undermining them all.
With that said, there were occasions when Walker had nobody to blame but himself.
The Liverpool howler was a moment of madness. Other times, his errors were less immediately noticeable. But in the lapses of concentration that allowed attackers to go unmarked and unchallenged, he was equally culpable.
As disappointing as these oversights could be, it should not be forgotten that Walker is still very much learning his trade at a relatively young age.
His decision-making might, at times, be questionable. His heart is certainly not.
That disappointing week or so in March undoubtedly hurt. As too, has some of the criticism that has come his way this season, something he alluded to in an interview with SpursTV (quotes via SkySports.com):
I set myself high standards last season and I'm pleased I did, but once you set those standards, if you then fall below them people start to get on your back a little.
Alone, these words might ring with a hint of self-pity. But Walker followed that up by acknowledging he was the "first to have a word with myself and say I 'could do better'" after "a bad game."
This spirit of determination was evident in the season's final couple of months when Walker worked as hard as (if not more than) anyone.
Against Basel (away) and Everton, Walker's grafting was very impressive. It did not pay off in wins, but in the absence of Lennon and Gareth Bale, he had done his utmost to provide his side a semblance of pace and penetration (while still doing his defensive duties).
If Walker is to stave off competition this pre-season from fellow right-backs Naughton and Adam Smith, a similar work-ethic will be vital.
Talent-wise, Walker may well be the best right-back in England, let alone at Spurs. He's already experienced highs and lows in his brief career, and now is the time for him to build on those experiences.
If he can, he will surely make the improvements that see him fulfill his enormous potential.