Is it still plagiarism if I didn't intentionally copy someone else's work and present it as my own, that is, if I plagiarized it by accident?
Yes, it is still plagiarism. Colleges and universities put the burden of responsibility on students for knowing what plagiarism is and then making the effort necessary to avoid it. Leaving out the quotation marks around someone else's word or omitting the attribution after a summary of someone else's theory may be just a mistake (a matter of inadequate documentation) but faculty can only judge what you submit to them, not what you intended.