[nigger in the woodpile] {n. phr.}, {slang} Something unexpected that changes a situation; a hidden factor or trick. — Racist and offensive, but commonly used in the past.
* /I knew there had to be a nigger in the woodpile, because the man was being much too generous./* /When the salesman gave him an extra tire for his bike, the boy suspected a nigger in the woodpile./