I'm not sure exactly why this is happening, so instead of linking you to a minimal sandbox I'll link you to the actual occurrence of the problem: http://cyclods.wikidot.com/thread:84

The "New post" button at the bottom of the page is added using this code:

[[image /main:posts/replybutton.jpg link="/post:new-post-%%name%%/edit/true/parentPage/%%fullname%%/tags/_open/title/Re: %%title%%"]]

That code is on a page called main:posts. The thread:_template page includes main:posts, and therefore it appears on the thread:84 page after going through the live template + an include. Not sure if that combination has anything to do with the problem.

The page's title is "Show/Hide content from specific users" … but somehow in the process of creating the child page with the same title (and "Re: " prefixed), everything after and including the forward slash ("/") is not shown.

i.e. instead of replies having the title "Re: Show/Hide content from specific users", they only have "Re: Show"

Edit: This was discovered almost two weeks ago, but I've only just gotten around to reporting it now ;-)