4.1. The Paradoxical Firewall
Many firewalls and other security devices have trouble identifying the Null Packet. Others claim to filter out Null Packets quite effectively and effortlessly. Interestingly, or not, both might be correct, which begs the omnipotence paradox: Can a firewall create a rule to filter out the Null Packet coming from the "outside", and not see Null Packets being allowed on the "inside"?
via RFC 6592