Not that this is such an issue, but strictly speaking, "self" should be used instead of "Piwik_Common" within the Piwik_Common class :
Piwik_Common::getCacheGeneral() Piwik_Common::initCorePiwikInTrackerMode() Piwik_Common::sanitizeInputValue() Piwik_Common::HTML_ENCODING_QUOTE_STYLE
Is there any reason to use Piwik_Common instead of self there ?
Please enlighten me.
Usually it's just a matter of personal coding style.
But if you refer to #2954, there are also backward compatibility issues.