I ran an update via Magento Connect and it messed up the whole site. I couldn’t access the frontend or backend – All I got was a White Screen Of Death on the homepage and a call to an undefined function in one or two of the PHP files, which, when fixed resulted in being able to navigate the Magento Connect Manager and see that there was an issue (as if I didn’t already know), but I still couldn’t use the admin section or the frontend.
The Cause
The apparent cause of this issue was when I changed some settings in Magento Connect Manager to allow ‘Beta’ release extensions to be installed (the default is ‘Stable’, naturally).
Unlike when you install a stable release, when I tried to do a beta release install, Magento seemed to download it, plus all the core files again (presumably beta versions of the core files i guess), which resulted in pandemonium and death for my Magento store.
The Fix
Still waiting on that one… If I find the answer I’ll post it here. If you know the answer, please post it here.
The Presumption
The consensus around the office is that this will most likely be specific to downloading older extensions that require modified core classes of a previous (older than v1.4.1.0 community edition) Magento in order to run properly and that the download of core files is triggered by the attempted install of an old or beta extension, which causes the new files to be overwritten and subsequently no longer work properly.
Advice
Follow common-sense, good practise when modifying Magento. Be careful when adding an extension – check that it’s certified for use with your version and if not, don’t try installing it on a production store without thoroughly testing on a development store first – of course this is obvious anyway, but even when you crash a demo install, it’s still monumentally frustrating 
.