Yes, accidental plagiarism is still plagiarism. If you accidentally use someone else's words or ideas without giving proper credit to the source, you have committed plagiarism, regardless of whether you intended to do so.
Common causes include forgetting to include citations, paraphrasing too closely to the original source without attribution, and losing track of which notes came from which source. Most academic institutions consider accidental plagiarism to be just as serious as intentional plagiarism. Our guide on accidental plagiarism covers the most common causes. Quetext's Plagiarism Checker catches both exact matches and paraphrased content so nothing slips through.