Before selling Cristiano Ronaldo to Juventus, the biggest mistake Real Madrid president Florentino Perez had ever made in the transfer market was allowing Claude Makelele to join Chelsea.
In the summer of 2003, the French defensive midfielder was seeking a pay rise that still would have only seen him earn half as much as his compatriot, Zinedine Zidane.
In an embarrassing show of ignorance, Perez utterly dismissed the merits of a player who was integral to how Real were playing at the time and deemed Chelsea's offer of £16.8 million for a 30-year-old far too good to turn down.
"He wasn't a header of the ball and he rarely passed the ball more than three metres," the Santiago Bernabeu supremo smugly reasoned. "Younger players will arrive who will cause Makelele to be forgotten."
The stupidity of the sale has never been forgotten, though, at least not in Madrid.
Bizarrely, it seems they have shorter memories at Chelsea, with whom Makelele won two Premier League titles, two League Cups and one FA Cup during six successful seasons at Stamford Bridge.
How else to explain the Blues' ageist and self-defeating policy of nothing more than one-year deals for players aged 30 or over?
Chelsea boss Maurizio Sarri has already publicly admitted he would scrap the restrictive stance if he could, given it has caused consternation with several members of his squad seeking contract extensions, including Willian, David Luiz and Cesc Fabregas, who has just departed for Monaco, leaving the Blues without a back-up for playmaker Jorginho.
More importantly, the west London club's reluctance to invest in players the wrong side of 30 is also frustrating the manager's attempts to add some badly-needed firepower to his team, in the shape of Gonzalo Higuain.
The Argentine turned 31 last month and, as well as being put off by the complexities of negotiating a deal with two clubs – Juventus own Higuain but he joined AC Milan last summer on loan for €18M (£16m/$20m) with a view to a permanent transfer for €36m (£32m/$40m) – his age is clearly an issue for Chelsea.
Their caution is understandable, of course. Higuain's very best years may well be behind him. Furthermore, in England, he is primarily known as a mentally fragile player who misses big chances in big games (a.k.a. the reason why Messi has never won a major international tournament).
Such a reductive appraisal does Higuain a great disservice. We are talking about one of the most prolific strikers of the past decade.
Indeed, since he first arrived in Europe in 2006, only Lionel Messi (369), Cristiano Ronaldo (343) and Edinson Cavani (232) have scored more goals than Higuain (214) across Europe's 'Big Five' leagues.
He has proven himself even more prolific than compatriot Manchester City's Sergio Aguero (202), who joined Atletico Madrid at the same time that Higuain linked up with Real.
Higuain may be 31 but there really isn't any other striker on the market with such a pedigree, as underlined by Real Madrid and Barcelona's struggles in finding a new forward.
Frustrated by the fees being asked for even Genoa's half-season wonder Krzysztof Piatek, the European champions have simply given up the chase, while their Catalan rivals have postponed the search for a Luis Suarez successor, and instead settled on an understudy, with Cristhian Stuani the leading candidate.
The latter may be 32 but he is in form and affordable, thus making him the best option for Barca. The same goes for Chelsea with Higuain.
In a winter window highlighting a dearth of promising young strikers, a forward with Higuain's recent record (the top scorer in Serie A since touching down in Italy in 2015) represents an excellent option.
However, it is not really about whether the stats suggest that Higuain is the right man for Chelsea. The point is that manager Sarri believes that Higuain is the right man for Chelsea.
And, if Chelsea believe that Sarri is the man to create a stylish and successful side at Stamford Bridge, then they must back him in the transfer market, particularly when it comes to addressing their biggest failing at the moment.
Alvaro Morata has failed dismally to score regularly in the Premier League, thus jeopardising Chelsea's hopes of a return to the Champions League and only strengthening the belief he is not mentally strong enough to be the leading man at a major club.
Of course, some will argue that Higuain suffers from the same syndrome. But nobody knows better than Sarri how to get the very best out of an equally complicated character.
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Higuain has long been known as an awkward player to manage. Even his agent and brother Nicolas has acknowledged that the forward is "introverted".
Therefore, it was a bold move by Sarri to tell Higuain on the first day of Napoli's pre-season training camp at Dimaro in 2015 that he was only playing at "70 per cent" of his potential.
"You're too lazy," the straight-talking, chain-smoking Tuscan declared. "If you don't change your attitude, you won't become the best centre-forward in the world."
Higuain responded by changing his diet, losing four kilograms (eight pounds) and scoring 36 times in 2015-16, thus breaking the goals record for a single Serie A season.
It was a stunning achievement but Sarri wasn't surprised.
"If he doesn’t win the Ballon d'Or in the future, then it'll be all his fault," he reasoned. "He'd be a right ****head."
The Ballon d'Or looks unlikely to happen at this stage but not because Higuain is not a bad guy, as Sarri well knows.
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"Up until now, he has projected an image which isn't in line with his character," he argued. "He's not only the best striker in the world, he's also a man with genuine feelings.
"Gonzalo can be a bit sensitive and it shows. But it's enough to strike the right chord with him."
Sarri clearly failed to do that with Morata but he would clearly have no such issues with Higuain, who has previously praised the Italian for providing him with "peace of mind".
It is not in the least bit surprising, then, that he is so keen on a reunion with Sarri after being shunted out of Juventus by the arrival of Ronaldo and then being left bitterly frustrated by Milan's maddeningly fluctuating form.
Sarri, for his part, would be getting a striker that he has the utmost faith in, one capable of providing the goals that Chelsea need to secure a top-four finish. In that sense, Higuain would be worth the investment.
His age may be an understandable concern. But countless thirty-somethings have proven inspired signings in the past: from Makelele and Gianfranco Zola at Chelsea to Higuain’s fellow Argentine attackers Gabriel Batistuta and Diego Milito, at Roma and Inter, respectively.
To paraphrase the old adage, if you're good enough, you're young enough. As Florentino Perez has now twice found to his considerable cost.