I logged in to the Piwik interface and saw the small message flagged up in the top right saying I should upgrade my 2.17 installation to 2.17.1. (I'm on PHP 5.4 so cannot go to 3.0.0 until the point at which we're able to move the installation to PHP 5.5+)

I clicked on teh message and then the automatic upgrade button (which has worked perfectly in the past) and then almost immediately got a message about an error due to the PHP version (see screenshot)

After digging into the files on the server, I discovered that the update has taken me to 3.0.0. Somehow it has been able to proceed with this bypassing the checks that it did previously and now leaving an unworkable system (until we can upgrade PHP).

I doubt many people will end up in this scenario, but it would be good to check that the version checking process works for any future cases where a higher specific version of PHP (or something else) is required and not necessarily available.