Renaissance

For over 500 years the statue of David has been a symbol of the Renaissance in Florence. Michelangelo's masterpiece is so famous, and has been studied so much for over 500 years that there couldn't possibly be any controversy over some detail, but it turns out there is.

What shape are the pupils?

It comes as such a surprise to many people they they are heart shaped, than it is being pointed to as a Mandela Effect, because they are sure they have seen the statue before with normal eyes.

Heart disease?

There's another anatomical anomaly regarding the statue. When a doctor, Daniel Gelfman, saw the statue in person he noticed a pronounced vein on the neck of David. He recognised it as a symptom of heart disease, having seen similar conditions many times in his surgery. Whilst some strenuous activity, such as singing or exercising, might make the vein bulge briefly, for one to appear so whilst at rest is a cause for concern.

The statue depicts David just as he is about to fight Goliath - he even has the stone in his hand - so it's possible this was a deliberate sign of his apprehension. This is unusual in itself, since most pervious works showed him after slaying the giant, sometimes even when he'd been beheaded.

Eye spy a mystery

There's no explanation regarding the reason the pupils were heart shaped. It's not even a common trend seen in other statues. With such phenomenal attention to detail Michelangelo showed in all his work, there would certainly have been a good reason for it, but it's lost to historians now. It's even been noticed he actually is slightly cross eyed, but this is put down to the fact the statue is so large and designed to be viewed from the bottom up, that the effect would be seen as him carefully surveying the scene of his upcoming battle.

Perceived changes in statues are nothing new to the Mandela Effect. It's happened with The Lincoln Memorial, and Rodins Thinker for example. 

Of course it could all be down to people just not noticing before, and no-one pointing it out to them, yet there are some who say they do specifically remember regular eyes.