Update: Apple re-releases OS X 10.6.5 server with Mail service fixes.
After releasing the latest version of OS X last week, Apple has today removed the OS X Server version of the update from its servers. When the updates were initially available, some people had issues with installers not working, but that was quickly addressed and since then the updates have been available.
According to a new report at AppleInsider, a number of people were having issues with the LDAP and DNS services after applying the update, and others have had problems with the management of external drives. According to the site, Apple confirmed it had intentionally pulled the update, but did not give a reason why.
If you are running OS X Server and have updated to the latest version of the software, you might want to double-check LDAP and DNS functionality, and if they are not working properly then you might consider downgrading to version 10.6.4 by using Time Machine if you have that configured on your server.
If you have not yet updated your Server software and have the 10.6.5 update available via a secondary software server or as a standalone download, then for now it may be best to avoid applying it.
The 10.6.5 update may have been ushered out the door since Apple began working on version 10.6.6 even before 10.6.5 was released, reportedly because of Apple's push to get support for the newly announced Mac App store into the OS.
Update: Apple has re-released the OS X Server 10.6.5 update, mentioning in an accompanying knowledgebase article that the initial release included a problem in the Mail service where a memory management problem would result in mail being sent to the wrong user.