Code comes from the codex:

<?php wp_redirect( get_permalink( $post->post_parent )); exit; ?>

I added the above, in an empty attachment.php page. When I visit an attachment page (eg. http://example.com/?attachment_id=2071) I get the error:

"This webpage has a redirect loop ERR_TOO_MANY_REDIRECTS"

Why?

Code comes from the codex:

<?php wp_redirect( get_permalink( $post->post_parent )); exit; ?>

I added the above, in an empty attachment.php page. When I visit an attachment page (eg. http://example.com/?attachment_id=2071) I get the error:

"This webpage has a redirect loop ERR_TOO_MANY_REDIRECTS"

Why?

UPDATE:

Based on the answer given, I updated my code to:

if($post->post_parent != 0):
    wp_redirect(get_permalink($post->post_parent));
else:
    wp_redirect(get_permalink(2));
endif;
exit;

That way if the post_parent == 0, it will go back to the home page. I don't know if I should be catching for anything else. But this seems to work.

Thanks Ignat B. for explaining what was going on. I have a few attachments that have post_parent = 0. And it just happened that the link I was trying out was one of them.