Chelsea and PSG made complete fools of themselves. It was brilliant.

Footballers -- and sit down and brace yourself, for your mind is about to be blown -- are very good at football. And the really good footballers -- like those out on the pitch last night for Paris Saint-Germain's hilarious humbling of Chelsea -- well, those guys are absolutely amazing at football. Ridiculously good. Best in the world, some of them.

So why do they do stupid things?

We're not talking about basic ordinary mistakes here, a pass that goes awry or a moment's lack of control. Football at the highest level is exceptionally difficult (to the point most attacking moves, even the good ones, end in failure) and, thanks to bad form or miscommunication or over-ambition, these things happen. But every now and then a footballer does something so inexplicable that no justification presents itself, and the only response is to blink, stare, wonder, and slowly shake one's head. Or laugh loudly, depending.

Football has a phrase for these, "moments of madness," and last night's extra time delivered two absolute beauties. In the first period of extra time, Thiago Silva and Kurt Zouma jumped for a header in the PSG box. Well -- Zouma jumped for a header. The Brazilian, who is PSG's captain and one of the most experienced and skilled defenders in the world, went up for a feeble impression of a volleyball block, lightly brushed the ball with his hands, and gave Chelsea a penalty -- and what should have been a winner.

You could see it on his face afterwards, the question and the answer, a two-line tragedy with just one character. "What the hell have I done, Thiago?" "Don't ask me, Thiago. I've no idea."

Speaking of excellent defenders, there is perhaps nobody on earth with such a rarified and advanced understanding of the art of penalty box interference as Chelsea's captain John Terry. Much of defending is simply about getting in the way, and nobody beats Terry when it comes to that. And as that last corner came over, there was Chelsea's captain, wrestling with -- hang on, that's his own teammate! They're marking each other! And they've left Thiago Silva free to loop his redemption over the goalkeeper! Oh dear.

John Terry marking Gary Cahill for Thiago Silva's equaliser last night. Effort, lads. pic.twitter.com/5lBhLeRwbG

— Squawka Football (@Squawka) March 12, 2015

Why humans make mistakes is a fascinating question with several answers floating around. A 2013 study concluded that people make mistakes not when the brain accidentally miscalculates, but when it receives flawed or "noisy" information and acts on that, a mental version of the 'garbage in, garbage out' principle. If so, one can only wonder at exactly the level of the noise required to make Silva conclude that the rules of the game had been temporarily suspended, or to make Terry and Cahill forget which colour shirt was the right colour shirt.

But whatever the reasons, one thing is certain: mistakes like these -- the proper clanger, the profoundly dropped bollock -- are one of the most enjoyable sights on a football pitch. After all, being good at something is as much about eliminating errors as it is about nebulous things like talent, and the best football teams tend to be those that make next to no cock-ups. So mistakes like these aren't just funny in themselves, though they are. They're also a reminder that perfection is not for the likes of us, even for the best of us.

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On top of the game, and despite their radically contrasting styles, Terry and Silva are near-immaculate. What last nights exchange of brainfades reinforced is the "near-"; there is within all humanity, even the most capable, a trace of inherent clownishness. Ordinary mistakes can be minimised, can be trained away, but however good you are, and however confident you feel, there is always the chance that the universe will, for no apparent reason and with no apparent warning, reach down and tie your shoelaces together. These aren't moments of madness; they're moments of mortality.

As such, these mistakes connect player to civilian, superstar to fan. Stick any member of the crowd on the pitch and they wouldn't be able to rise, salmon-coloured, and reproduce SIlva's winning goal. But they might well end up misplacing their mind and slapping the ball away, or accidentally blocking their own teammate at a corner.

As the old saying goes, "To err is human, and to err really, really embarrassingly on international television at a crucial moment in your career is just as human, but much funnier, and we'd like to see more of it, please and thank you."