Salting the soup, while sipping the spoon

To not understand something correctly or what is important about it especially when it is obvious to those around you.

No matter how much salt you put in the pot it will not change the spoon.

"You can't use that as an argument, you are salting the soup, while sipping the spoon".

"I was trying to make soup, but then I started salting the soup, while sipping the spoon".