There are many places where two gospels differ from each other and nobody can say it is a corruption of one of them.
"But of that day and hour knoweth no man, no, not the angels of heaven, but my Father only." Matthew 24:36
"But of that day and that hour knoweth no man, no, not the angels which are in heaven, neither the Son, but the Father." Mk 13:32
So how will you decide, which variant is the right one for Matthew 24:36?
TR: "But of that day and hour no one knows, not even the angels of heaven, but My Father only." Matthew 24:36
NA27: "But of that day and hour no one knows, not even the angels of heaven, nor the Son, but My Father only." Matthew 24:36
According to your logic, the variant with "nor the Son" is the right one, because it is also in Mark this way.
In that case you have error in your KJV, because it is based on TR without this clause.