When Manchester United ponder where it went wrong last season, the natural instinct is to head straight to the moment Vincent Kompany outjumped Chris Smalling to head the only goal of April's Manchester derby
Sir Alex Ferguson's team selection, and his mistaken belief that Park Ji-sung could nullify Yaya Toure, soon springs to mind, too.
Yet perhaps more costly in the final reckoning were events at Old Trafford eight days earlier.
Leading 3-1 and 4-2 against Everton, United surrendered their advantage, losing two goals and the two points that brought Manchester City back into the equation.
"It definitely cost us the league," Ferguson reflected this week. "If we are 4-2 up with seven minutes to go this time it won't happen again."
The architect of their downfall was clear to see.
Scorer of one goal, involved in two more and the most immediately identifiable figure on the pitch, Marouane Fellaini began to assume a status as United's tormentor.
"We just didn't deal with the balls into Fellaini, it's as simple as that," Ferguson lamented.
They did not again when Everton won 1-0 at Goodison Park in August.
"They just lumped the ball forward to him, that's all they did," Ferguson complained.
But against a United side reduced to one solitary specialist central defender by injuries, it worked.
Bizarrely, Michael Carrick was assigned to mark the big Belgian at set pieces. Predictably, Fellaini headed Everton's winner from a corner.
If it gave Everton the status as United's bogey team, the No 25 started to seem their nemesis.
United have twice lost in a game of trial by Fellaini.
So the cheers from Manchester may have been heard on Merseyside when a hip injury rendered Everton's talisman a doubt for Sunday's game.
The bad news for United is that manager David Moyes expects him to be fit.
Compared to August, Ferguson has more muscular and battle-hardened defenders available again - and, especially if Fellaini is fit, there is a case for reuniting the old firm of Rio Ferdinand and Nemanja Vidic, sorely missed against Everton 10 months ago - for a test that is part technical, part physical.
United have failed two such already, at Goodison Park and at home to Tottenham Hptspur, whose pace and power in midfield unsettled them.
Fellaini stands out, not merely with his jutting elbows and unruly hairstyle, because he is precisely the sort of midfielder (though he is an auxiliary attacker these days) that Ferguson is reluctant to sign. He has overlooked both sizeable, forceful players and specialist defensive midfielders to focus on smaller, slighter talents.
In January's rematch against Tottenham, centre-back Phil Jones was shifted forward to give United a bulk they otherwise lack.
With Ferguson, having configured teams to stop Liverpool and Tottenham of late, it will be instructive if he selects a United side today with greater height than usual.
While Moyes has his smaller, slighter players, the gifted trio of Steven Pienaar, Leon Osman and Kevin Mirallas in particular, Everton's aerial threat is increasing.
With Nikica Jelavic's goals drying up, Moyes has made the giant Victor Anichebe his first-choice forward.
Aston Villa could not cope with a legal form of bullying last weekend with Everton, as they did at Old Trafford last season, coming from two goals behind to procure a point.
"They are obviously hard to beat," Ferguson said.
"They have only lost three games [all season]. That is in the mould of David Moyes."
United's rally this season is in the mould of their manager, too.
Since their opening day defeat at Goodison, they have met every challenge.
For them, this season is about righting the wrongs of last, about correcting the mistakes.
This is a time to see if lessons have been learnt, to tell if they can avoid a repeat of their collapses, to Everton over seven minutes and City over six games.
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