Plagiarism may be:
The consequences for plagiarism apply even for unintentional plagiarism.
(From Victoria University)
"Generally speaking, you can regard something as common knowledge if you find the same information undocumented in at least five credible sources. Additionally, it might be common knowledge if you think the information you're presenting is something your readers will already know, or something that a person could easily find in general reference sources. But when in doubt, cite; if the citation turns out to be unnecessary, your teacher or editor will tell you."
(From Purdue OWL)