PHP Notice error (when on 404 page)

The 404.php template file is used for 404 errors: i.e. no posts found.

Since there are no posts, there is no $post object. With no $post object, functions such as the_ID() are not available, and will return the error you’re observing.

The fix: replace all instances of $post-derived data with static data. e.g. replace this:

<div id="post-<?php the_ID(); ?>" <?php post_class(); ?>>

…with this:

<div id="post-0" <?php post_class(); ?>>