when you remove the reference, phpbb removes the attachment, it seems to be a bug... as it's no longer displaying it but it is trying to, even though there is no longer any reference to it in the post.
Root cause is likely that we are using old templates on a new version of phpbb.. so tricky to resolve without making a new theme to the forums.
Moral? I guess don't edit posts of attachments, just work with the way phpbb wants to do it.
There's a few icons in the left margin I need to correct, too, I see.. case of game or fix forums... hmmmm