No that's not what happened at all.
You deleted the placeholder Hello World post and as soon as you publish a post not a page and the error message will be gone. http://en.support.wordpress.com/post-vs-page/
By default the front page of your blog displays all posts (not pages) in reverse chronological order, with the most recently published post on top. The 404 (page not found) error message on the front page of the blog appeared because the Admin of the blog deleted the placeholder Hello World post. http://en.support.wordpress.com/not-found/