The Scribe's Error
A scribe copied a manuscript. He made one error — a single transposed word.
The next scribe, copying the copy, saw the strange phrasing and "corrected" it in a way that made sense to him.
The next scribe saw the correction and adjusted the surrounding passage.
Each scribe made the text a little more consistent with the errors before.
Three centuries later, scholars found the original. They accused it of being a corrupted copy.