Had the same happen, searched around and it seemed to be caused by the Gmail feature that automatically creates events from invitation emails you receive, even

Had the same happen, searched around and it seemed to be caused by the Gmail feature that automatically creates events from invitation emails you receive, even if they land in spam. Spammers seemed to be using that to their advantage, so I just turned the feature off. EDIT: The original article covers this and more, go read it :)