Testing Piwik with XAMPP on XP Pro with PHP 5.3 there is a warning
Notice: Undefined index: Core in \piw\core\View.php on line 219
as soon as browser opens a page in Piwik.
Browser: Firefox 3.5.6 with several developer extensions.
With other systems using PHP 5.2.x this warning is not visible. But, even there there is still trouble when trying to track my-domain.xy, a.my-domain.xy, b.my-domain.xy and so on.
It still does not count correct:
Site 1 is counted with incorrect number of impressions (far to few), site 2 is tracked occasional, site 3 is normally not tracked, site 4 and later makes this even more troublesome.