there is a fine line between (something) and (something else)
There is a very narrow or indistinct division between two deceptively similar things, one of which is worse than the other. As a new parent, you learn that there's a fine line between looking out for your child's welfare and being overprotective. A: "I only borrowed your sweater, I swear!" B: "Well, there's a fine line between borrowing and stealing, isn't there?"
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