Lectio difficilior potior (Latin for "the more difficult reading is the stronger") is a principle of textual criticism. Where different manuscripts conflict on a particular word, the more unusual one is more likely the original. This is because scribes would more often replace odd words with more familiar ones than vice versa.

This principle has some wider everyday application. If one wants to determine the correct spelling of a name, and finds conflicting versions, it is often the more "difficult" one that is correct, not the one that is most widely used. For example, a British politician was correctly named Peter Alexander Rupert Carington, Baron Carrington - the family name has only one "r", the peerage title two. However, a Google search, which can often be useful to determine such matters, might find:

"Peter Alexander Rupert Carington" - 32 hits
"Peter Alexander Rupert Carrington" - 79 hits

Choosing the "more common" spelling would thus be wrong in this case. However, even without definite knowledge of what the correct spelling is, Carington is to be preferred because it is clearly the more unusual. If Carrington were correct, there would hardly be such a high incidence of the particular misspelling Carington. But the reverse is not surprising, since people might easily consider the unusual name Carington a mistake and falsely "correct" it.