I have 2 separate installations of Magento in different sub-domains on the same host.
The first one, my development store where I test upgrades, try out new add-ons etc. upgraded from 1.8.1 to 1.9.1 just fine (I used the upgrade in Magento Connect) apart from a couple of minor colour issues in a 3rd party template which were easily fixed.
I have just tried to upgrade the live store using exactly the same method and it crashed out part way through (got a cache flushing error in the Magento Connect console) and now I am getting an invalid PHP version error (says I need PHP 5.3.0 or newer). I have run phpinfo and I am getting 5.2.6 as the installed version.
This makes no sense at all, as the test store is running just fine with 5.2.6.
Anyone have any ideas?