I've noticed if I go with
domain.com/post_id/
then typing anything else after the slash after post id will still take you to the origional
domain.com/post_id/ url.
it's also vice.versa.
if you have
domain.com/post_id/anything
as your permalink, it'll still be functional if you leave "anything" out of the url.
are these problems for SEO worries?
redirects?