... if my friend did it (or a powerful person I want to defend)
if the accuser is a jerk
if everyone does it anyway
if the source is wikipedia
if it was unintentional, inadvertent
if it only occurs x many times in a book of y pages
if the plagiarist doesn't do it all that often
if it wasn't the most important part of the argument: just "ancillary".
it is just the matter of missing quotation marks and references
if the plagiarist was in a hurry
if it's just "intertextuality"
it is "unoriginal genius"