Upgrading a customized Community Server site is always a real pain-in-the-ass an exciting adventure, because you never know where you'll find a new reaming opportunity to learn something new!
The client sites I upgraded from CS2008.5 to CS2008.5 SP1 today were only moderately customized. They did contain several original pages and custom Chameleon controls that require knowledge of their context, like all Chameleon controls do. If the control doesn't know its context you get something ugly like a null exception when you need an application object.
One of the ways to establish context is with an ?App=$1 query string specified in the SiteUrl.config url vanity property. The SiteUrl_Override.config urls for my original pages based on the ?App= query logic no longer knew their context because the App query string did not exist, because CS2008.5 SP1 now uses ?GalleryApp=$1 instead.
Below you can see the CS2008.5 version is in red and the SP1 new in blue (kinda like the recent election results.)
