Football fans in England found out the hard way what happens when Cristiano Ronaldo is booed, shortly after the 2006 World Cup finals.
The Portugal international was public enemy number one following his involvement in Manchester United teammate Wayne Rooney's sending off for England in the World Cup quarter-finals; but each time he was booed, jeered and abused, he would respond by defiantely improving his performances.
In the end, by booing the dangerous Ronaldo, supporters were doing far more damage to their own teams than the gifted winger, who seemed to thrive off the negativity.
Seven years on and Ronaldo still takes his fair share of stick from opposition supporters, although for some years now the boos have made way for antagonistic chants hailing his direct rival for the title of the 'world's best player': Barcelona's Lionel Messi. But it has the same effect.
The Argentina international has won the last four Ballon d'Or awards and, perhaps up until last night, is widely regarded as the finest player of his generation.
Messi and Ronaldo are very different - in terms of both their playing styles and personalities - which is why their ongoing duel for the world's best player crown is so enticing.
Everyone has an opinion as to which player has the upper hand, but there is no definitive answer, which means the debate is likely to last well after both have hung up their boots.
But maybe one day the two players will openly admit that their indirect battle improved them both as players.
Certainly, it's difficult to imagine that Ronaldo would be the same devastating player he is today were Messi not on the scene.
Even as a Manchester United player, Ronaldo made it clear to his teammates that he was intent on become the best player in the world. And the amount of hard work and dedication he's put in over the years shows he wasn't prepared for his lifetime ambition to remain a pipe dream.
In order to achieve this he needed to match Messi's phenomenal goalscoring exploits - and that's precisely what he's managed to do over the course of 2013.
Right now he's playing the best football of his entire career. He's not been content to play second-fiddle to Messi, and it's hard to argue that the Portuguese superstar is the world's top dog footballer at this moment in time.
If he wins the Ballon d'Or award in January, it will be fully deserved, and not even the Barcelona contingent could begrudge him his moment in the spotlight.
And although he won't mention Messi in his winners' speech - the Argentina international has undoubtedly had a massive role to play in Ronaldo's considerable improvement since 2009.