Apple Books slaps an attribution notice on the end if you copy 4 or more words from a book. The Verve got sued by The Rolling Stones for a 4 second sample on 'Bittersweet Symphony'. Post 'Blurred Lines' you can now be sued for copying "the feel" of a song.

Really what it comes down to is do you have enough resources to convince a judge or jury that X is a copy of Y? Doesn't really matter the size of X.