When posts get moved, someone replying during the move replies and the reply becomes detached and belongs to the original forum still. If the new forum is hidden, users can search and still access to the reply, and in threaded mode see the subjects of the posts in the forum they don't have access to.

Here is a select I made to check how many posts were suffering from main and replies having different forums:

select r.B_Board, m.B_Board, r.B_Parent, r.B_Main, r.B_Number from w3t_Posts as r, w3t_Posts as m
WHERE r.B_Main = m.B_Number
AND m.B_Topic = 1
AND r.B_Board <> m.B_Board;

I'm just wondering if many other boards that do a lot of moving of posts from forum to forum have this problem?

It has happened a couple of times at my site, but we only move threads between open forums so it just means that it isn't possible to link directly to the post, I think. Anyways, when it has happened I've just updated the board keyword.

Taht was supposedly fixed in 6.3 I believe. Need to check the changelog - what version are you noticing this under?

6.2 still.

I didn't have a copy of a few of the 6.2 versions -but there's a check to make sure that the parent still exists in addpost.php in 6.2.3 so it must have been added somewhere between 6.2 adn 6.2.3.