your post made me look at the diff for the last update to filter.inc (the CARP related one), and indeed, it appears to cause the problem.
The lines:
if ($int != false and $int != $wan_interface) {
3169 3168
$ipnet = convert_ip_to_network_format($ip, $carp['netmask']);
3170 3169
if($int)
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$lines .= "nat on {$int} inet from {$ipnet} to any -> ({$carp_int}) \n";
I changed "if($int)" to "if($int!='gif0')" and the error goes away. Now I realize this is an ugly hack, but I think it proves that there is an issue here. Will file a bug report.