Here’s a fascinating article on the psychology of plagiarism, particularly how the plagiarist’s ignorance of his own ignorance is often his undoing: The Mind of the Plagiarist:
It never occurs to the plagiarist, therefore, whether panic-stricken or calculating, that submitting someone else’s prose under his own name might alert a wary reader that shenanigans are in play. It never occurs to him that a vapid three-sentence paragraph of his prose, with simplistic sentences, bad grammar, and misspellings, when followed by a paragraph in a competent writer’s professional prose will create a sense of disjunction in the very party whom the gesture aims at defrauding.
If there were such a thing as an intelligent or well-educated plagiarist, the idea of a careful patchwork of paragraphs, culled from various websites and rewritten to make the style homogeneous and framed within original prose that endowed on the whole something like a convincing structure — that, I say, might occur to him. But if the plagiarist were intelligent and well educated, if he were that capable, he would probably not be a plagiarist; he would be an honest student who acquits himself in courses.