I propose to look at plagiarism as a system or a structure, not an isolated, accidental event. People often get distracted by wondering whether an act of plagiarism was intentional or not. They look at the issue of mens rea. This ends up being confusing, because a lot of plagiarists do not commit this act intentionally.
What I mean by structural plagiarism is the following. This poet who had plagiarized very extensively wrote down other peoples poems in her own notebooks, along side her own work, and often reworked someone else's poem extensively over a long period of time. Then, when sending out work or assembling a collection of poems, she sometimes used a poem that was not altered very much at all. This is a virtual recipe for plagiarism. It is like littering a path with banana peels, walking down the path, and then calling your falls "accidents." Maybe you did not intend to fall, and tried hard not to slip on those peels, but you did intend to litter the path with banana peels or ball bearings. The fault is one stage earlier in the process, but it is nonetheless your own fault.
Ira Lightman says that "Plagiarists never do it once." This is because plagiarism is the result of a method of writing in which the writer refuses to keep track of what words are his or her own.
Now I can see someone accidentally not closing a quotation mark at the end, or slipping up in a minor way with a brief sequence of quoted words. I think this kind of mistake, though, is not greeted with howls of outrage. What makes people made is the serial plagiarist, the person who makes a systematic practice of laying down banana peels on the road and slipping on them.