Wiki edited by Laura Gibbs.
In my experience, people usually don't consciously decide to plagiarize, but they may end up plagiarizing "by accident" because they run out of time, or they get confused about the assignment, or maybe they copy-and-paste, intending to go back and edit later but forgetting to do so. Every time that I have seen plagiarism in an assignment, the person swore that the plagiarism happened "by accident." That does not change the fact of the matter: plagiarism, even when it happens by accident, is still plagiarism, and the consequences are serious. It's like when you are caught speeding or running a red light: it doesn't matter if you did not know you were speeding or if you did not notice the red light - you are still going to get a ticket.
Via Dennis T OConnor, Rui Guimarães Lima, Paulo Tomás Neves