One of the most important tasks of historians of scholarship is to ascertain the date at which certain texts became available to the public and, if possible, also to establish who actually owned them and when and how they were collected. Readers of manuscripts are in continuous debt to the compilers of catalogues. However, mistakes in catalogues can have a long life and they can lead generations of later researchers into error. This is illustrated by a small group of manuscripts which, owing to catalogues, has, for centuries (because of catalogues!), been associated with Josephus Justus Scaliger (1540-1609), whereas they actually belonged to Franciscus Raphelengius (1539-1597). The author also discusses the study of Arabic in Northern Europe during the Renaissance.