Lectio difficilior potior (Latin for “the more difficult reading is the stronger”) is a main principle of textual criticism. Where different manuscripts conflict on a particular reading, the principle suggests that the more unusual one is more likely the original. The presupposition is that scribes would more often replace odd words and hard sayings with more familiar and less controversial ones, than vice versa. … Erasmus expressed the idea in his Annotations to the New Testament in the early 1500s: “And whenever the Fathers report that there is a variant reading, that one always appears to me to be more esteemed (by them is the one) which at first glance seems the more absurd-since it is reasonable that a reader who is either not very learned or not very attentive was offended by the specter of absurdity and changed the text.”
From Wikipedia.