Plagiarism can occur accidentally because of problems with using

You work with someone else on a prescribed essay topic you are both doing, and take shared notes, with the result that each of you is drawing on the same words and phrases. Then you both submit essays that look as though one is copied from the other. In this case, you should protect yourself by indicating in the essay when ideas and words come from a shared discussion or from a process of collaborative research.

The internet, books and anything else another author has published. Plagiarism charges are so simple and can happen if you don't quote your place of findings.