I recently installed the Joomla user vulnerability patch (it was leaked prior to the release of 1.5.6) to this site. I didn't upgrade from 1.5.4 to 1.5.6 because I was in a hurry (naughty me!)

So I secured the site, but finally today I have been prodded in to upgrading, only to discover an interesting 'feature' of 1.5.6:

I have a CPanel level redirect on http://joomladay.org.au/forum to our community forum. This is because I created a subdomain http://forum.joomladay.org.au and it auto-creates the /forum directory. I have a redirect on the subdomain to the correct forum Joomla URL.

With native Joomla SEF URLs switched on, this worked fine in 1.5.4.

In 1.5.6 it stopped working. I couldnt figure out why the menu link, which now resolved to http://joomladay..org.au/forum had suddenly lost its redirection.

It's because it had (I finally discovered) set the alias in the forum meny entry to 'forum'. So this was arriving at the /forum subdirectory and dying (because this is a dummy sub-directory used only for the subdomain - it's an empty directory).

Anyway, to cut a long story short, I went in to the menu entry and changed its alias to 'community_forum' and everything worked again.

I mention all this here in case someone else has a similar problem.

Watch out for system level (.htaccess) redirections clashing with your 'new' menu aliases.