It seems that sinning is one thing, but sinning and saying God is wrong about that being a sin is, at a minimum, guilt for 2 sins instead of just the one originally committed.
Sin #1 + Sin #2 - Calling God a liar/deceiver.
There’s no other way to view “God is wrong” than it being another way of saying He is a liar. Since He is all knowing, then He would know if something is not a sin but He goes ahead and tells everyone it is a sin, then He is lying and deceiving.
Since I have limited knowledge, it is possible for me to be wrong and not be a liar. However, God can’t be wrong, so any “mistake” He makes is really intentional.
Saying God is wrong is a grave sin, in my opinion. It is rebellion.