there is a fine line between
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?"
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
there is a fine line between (something and something else)
There is little difference between something and something else. There is a fine line between a frown and a grimace.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.