In an interview with Preaching Magazine, preacher, professor, and author Haddon Robinson warns against the danger of legalism:

When an abstract concept says, for example, "You shall honor your father and mother," it's clear that's what you're supposed to do. But then I apply it to my aging parents. When my father came to live with us, he lost touch with reality, and we had to put him in the nursing home. I hated to do it, because he didn't like to be there. But when my wife's mother came to the end of her life, we kept her in our home, and my wife took care of her.

It is very easy to come to the conclusion that if you are going to honor your parents, then you must keep them in your home when they get old. But there was a different situation—our kids were grown when Bonnie's mother was ill. In both cases I was trying to honor my parents.
Legalism is when the application of a principle is given all the force of the principle itself.