Customarily, Middle English scholars search for texts to discuss through either manuscript catalogues or their derivatives, bibliographical indexes. However, both procedures are only approximative; as M. R. James pointed out a century ago, one should never trust that a cataloguer has told you everything. Moreover, unique Middle English texts routinely occur in MSS many scholars think do not require examination, e.g. those written in Latin or copied in the early modern period. This essay collects a number of examples from Oxford libraries, at least one of which challenges conventional literary history accounts of Middle English literature.
Ralph Hanna (Sun,) studied this question.