The (new) check for the default character set in 3.3.3 just looks for the string 'UTF-8' and if it isn't found a red error message is displayed at ACP's General tab.
This happens even if this is the default char set but the PHP variable is spelled in lower case (which was the case when I updated from 3.3.2 to 3.3.3).
By changing acp_main.php, line 707 from
'S_DEFAULT_CHARSET_FAIL' => $default_charset !== 'UTF-8',
'S_DEFAULT_CHARSET_FAIL' => strtolower($default_charset) !== 'utf-8',
this error message could be prevented