Deut 11:16-17 - The path to unyielding ground
So Moses told all the people that if they obeyed God they would receive some crazy cool blessings, but that when those blessings became reality they needed to pay attention to their hearts and not let them be deceived, because those very blessings would become temptations which could pull their affections away from God and onto them. They needed to understand that the temptation to focus on the "stuff" of God as opposed to God Himself would always be there, so they needed to guard their hearts from this, otherwise there would be devastating consequences. The way to "beware" is to simply Be Alert, to Be Ready. The same command Jesus gave to his disciples when they asked Him about the End Times - He simply said "just be ready, don't be caught off guard."
What's the penalty for allowing your heart to be deceived and then following after other gods (stuff)? God gets mad at you and you get the same curse that Cain got! The ground will stop yielding! No matter how hard you work you will not reap the reward you seek. You may make good money, but it won't satisfy you. You won't be able to yield that which nourishes your soul. Verse 17 says "the ground will not yield it's fruit...." - You won't be able to eat the product of your labor. You will have no sustainability to keep working. Why? Because there's no food! If we want to eat we need to work, but we see in this verse that if we disobey God we will work but we won't get to eat! The nourishment that we need will evade us. This is why Cain said his curse was unbearable. His means of sustainability was gone! And this is the curse for serving anything other than God alone!