So What Happened?
The painting is so well known, you might not be surprised to learn that it crossed paths with two of history's most famous figures. But, you probably didn't know that both figures went out of their way to treat it like it had been pooped up there by someone with a remarkably imprecise and explosive strain of diarrhea.
The first noted figure was Leonardo himself. See, frescoes are supposed to be painted on wet plaster or they start to peel. Unless you want your fresco to have the shelf life of a Banksy painting, the wet plaster is as important to the equation as the paint. But this wasn't any painter, he was Leonardo motherfucking da Vinci and if he wanted to paint on a dry surface, who's going to stop him? Unsurprisingly, the whole thing turned to a horrible peeling mess within his own lifetime.
By 1652, it was considered ruined and unrecognizable. This may explain why someone decided later to knock a hole right through the base of Jesus to make room for a door.
Just so we're clear how much of a mess this was, this is the 15th century Catholic Italy we're talking about. Jesus was sort of a big deal over there.
Then Napoleon, a great lover of art and anything Leonardo, came along and decided it was shit. When his troops were stationed in Italy, he decided to turn the whole room into a storage closet. Then, because he wasn't quite finished dragging his ass all over Leonardo's second greatest work, he decided to turn it into a prison.