nigger in the woodpile
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.
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Source: A Dictionary of American Idioms