It was Ederson's mistake when conceding the second goal in Manchester City's 2-1 defeat against Shakhtar Donetsk on Wednesday, which underlined the impact that the Brazilian goalkeeper has made since arriving at the club during the summer.
The 24-year-old marked his debut against Manchester United, during an International Champions Cup friendly in Houston in July, by making an almost identical error of judgement which led to Romelu Lukaku's goal in the NRG Stadium, prompting suggestions that Pep Guardiola had made another bad call (after Claudio Bravo) in identifying a No. 1 goalkeeper.
Yet in between Houston and Wednesday night in Ukraine, the £35 million signing from Benfica has been hugely impressive for City and, in many ways, proved himself to be as crucial to Guardiola's team as David De Gea has become across town at United.
For all the focus on the attacking talent and star quality of each squad, it is arguably the two goalkeepers who make the difference for City and United.
De Gea's incredible performance during United's 3-1 win at Arsenal last Saturday was merely the latest example of the Spaniard's importance to Jose Mourinho's team.
The 27-year-old has now earned comparisons to United's great keepers of the modern era, Peter Schmeichel and Edwin van der Sar, with his performances. And that's a remarkable situation for De Gea considering the difficulties he experienced during his early years at Old Trafford after being signed to replace Van der Sar, as a raw and skinny 20-year-old, in 2011.
United without De Gea, for all of Sergio Romero's qualities as his understudy, would be a much less formidable outfit and the same can now be said about City and Ederson.
Ederson has still to produce the kind of match-winning performances that De Gea regularly carves out for United, but he has brought stability and certainty to the goalkeeping position for City and that, in itself, is a mark of progress.
Last season, with Guardiola instantly discarding England goalkeeper Joe Hart, Bravo was signed from his former club Barcelona to play the role of sweeper-keeper, but the Chile No. 1 proved a disastrous signing, with a series of high-profile mistakes costing City dearly — most notably on his debut when he dropped the ball in the build-up to Zlatan Ibrahimovic's goal as City lost the derby 2-1.
Bravo's failure to adjust to the Premier League affected the confidence of his defenders and City became a soft touch during the middle part of the campaign. But Guardiola identified the problem in goal and Ederson has clearly been an upgrade.
The Brazilian has also enabled Guardiola to alter City's style of play, despite the coach insisting last season that would not compromise his principles. City have become a more direct team going forward due to the keeper's ability to turn defence into attack with his pinpoint distribution, both with his hands and feet.
Guardiola admitted during the summer tour of the United States that his interest in Ederson had been sparked by the goalkeeper's performance for Benfica against his Bayern Munich team in the Champions League quarterfinal in April 2016 — which the German side eventually won 3-2 on aggregate.
Ederson's kicking, and the length he could clear with his goal-kicks, prompted Guardiola to spend an hour with his coaches, discussing how to counter the threat posed by the Brazilian's clearances.
His presence in goal allows City to hit teams much more quickly on the counter-attack, exploiting the pace of Leroy Sane, Gabriel Jesus and Raheem Sterling, and it will be a key tactic against United at Old Trafford on Sunday.
Of course once City get behind the United back line, they will still have find a way past De Gea.
Against Arsenal, the former Atletico Madrid keeper made 14 saves — the most ever recorded in a Premier League game — and his performance was as crucial to United earning three points as the goals scored by Jesse Lingard (2) and Antonio Valencia.
In the direct head-to-head between De Gea and Ederson, the United keeper has greater experience and is less prone to the kind of mistakes which marked his early years at the club.
Ederson, as the error against Shakhtar highlighted, can still be caught out, but he is a rising star in the goalkeeping world and his success in quickly settling into the Premier League has largely gone unnoticed.
On Sunday, in the pressure cooker environment that is the Manchester derby, both men will be crucial to their team's hopes of victory.
Mark Ogden is a senior football writer for ESPN FC. Follow him @MarkOgden_