"You should seek after things with eternal significance."
Why this belief is harmful: On the surface, this sounds like Jesus' "Lay up for yourselves treasures in heaven." But rather than meaning that you should help people and seek spiritual meaning, instead of hoarding stuff for yourself, the way it was taught to me in Mormonism is that you should never trust your own judgment about pretty much anything. It doesn't matter if something seems harmless, or healthy, or even necessary to you or others. If it's not going to be around forever, or be okay to have or do or be with in God's Celestial Kingdom, then it's worthless.
This can have horrifying consequences.