Forgotten how to fix this:

This has always been an error on my board but forget to fix it until now (but now can't find the post that tells me what to do):

When I get a thread title with "new post" on the end I get a problem with the redirect.
EG:
http://www.domain.com/f12/435345-my-...-new-post.html

What is the solution?

You can change either the position of the new-post variable, ie /456-new-post-my-thread.html or use something else, such as 'newp' that isn't a likely end of thread word.

click advanced options + sign to expand under the presets.

find

'Newest Post in Thread' Redirect

and edit the setting to a custom method as you see fit.