Yes, forgetting to cite a source is still considered plagiarism. Plagiarism is the act of presenting someone else's work or ideas as your own, whether intentionally or unintentionally. When you do not properly cite a source, you are not giving credit to the original author for their work. This can have serious consequences, such as a failing grade on the assignment or even expulsion from school.
It is important to always be careful when citing sources. If you are not sure whether you need to cite something, it is best to err on the side of caution and cite it. There are many resources available to help you with this, such as online guides and citation generators. By taking the time to properly cite your sources, you can avoid plagiarism and protect your academic reputation.