a hair in the butter

A problem or challenging situation (likened to trying to pick a single hair out of a slippery substance like butter). Things were going smoothly enough on our road trip until we found out that every hotel in town was booked for the night. Talk about a hair in the butter! Forgetting my textbook at school has proved to be a real hair in the butter for my study plans. A: "You want to borrow the car? Sure, go ahead—it just doesn't have any gas in it at the moment." B: "Well, that's a hair in the butter, isn't it?