404 Error page appears incase of missing contents or pages in blog. If a page is not found then Blogger shows an error. Blogger recently introduced a new feature to create your own 404 error page for your blog.
My 404 Error

Blogger's default error is as below
"Sorry, the page you were looking for in this blog does not exist."

To change your 404 Error, simply Log in to your Blogger account. (you must be using the new Blogger dashboard)
Go to Settings > Search preferences > Custom Page not Found. > Click Edit to open the text box. Type whatever you would like to appear on the page, then click on "Save Changes".
Now here you can put whatever you want to show in your 404 Error... You can use html tags.