It's not plagiarism

... 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"

etc...