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Wikipedia is a free source of knowledge, and people are encouraged to reuse content from Wikipedia. There are several publishers that mirror or print Wikipedia content with proper attribution (e.g. Books LLC). However, a few have copied content from Wikipedia without attribution. One problem with this is that it's unethical as the original Wikipedia contributors are not given any credit for their work. But there's another bigger problem: several of these plagiarized articles or books are later added to Wikipedia articles as references, thus introducing the problem of circular references, also known as citogeneses.
The following sources plagiarize content from Wikipedia, and should not be used as references. Please feel free to add to the list, with appropriate revisions of the Wikipedia articles as evidence.