The first false assumption implicit in a firewall system is that there is only one way into a network, so that you can focus all your security attention on that one point.
Reality is that there is an entry into the network for every host on the network.
According to surveys, the majority of computer crime is committed by insiders. This can be read as implying either that external computer security at most corporations is so good that most of the time, the outside bad guys can't get in, or that the internal security is so poor that it's easier for insiders to commit computer crime. There are those who disagree with me, of course. So, for example, if you're moving sensitive data, your protocol should include some kind of encryption to prevent effective interception of the data. If data integrity is particularly important, then the protocol should include a CRC on the data.