If it hits the post and goes in, goal, if it hits the post and doesn't go in, its no goal, its very black or white that, more so than many other decisions (laws).
But Aguero’s penalty did go in so surely that can’t be ruled out?
Another scenario - the penalty is on target, the goalkeeper pushes onto the inside of the post and then it hits his hand a second time and goes into the net. There’s no way that could disallowed but by the example above it would be. It would cause mayhem.