It seems if GPC_order is set to CGP and get-variable
is a scalar but post-variable is an array then final 
value will be an empty array.

For example if test.php3 is

<?php echo "a = $a, count = " . count($a) ?>

then result of form   

<FORM ACTION="test.php3a=blah" METHOD="post">
<INPUT TYPE="hidden" NAME="a[]" VALUE="1">
<INPUT TYPE="hidden" NAME="a[]" VALUE="2">
<INPUT TYPE="submit">
</FORM>

is "a = Array, count = 0" instead of "a = Array, count = 2".