a thin line

A very narrow division between two alternatives, one of which is much worse than the other. As a new parent, you learn that there's a thin line between looking out for your child's welfare and being overprotective. A: "How could I possibly have a crush on Brian? He's such a jerk! I can't stand him." B: "Yeah, but there's a thin line between love and hate." The film treads a thin line between parody and homage.