One of your internal MASQed machines are creating an abnormally high number of packets destined for the Internet. As the IP Masq server builds the MASQ table and forwards these packets out over the Internet, the table is quickly filling. Once the table is filled, it will give you this error.
The only application that I have known which temporarily creates this situation is a gaming program called "GameSpy". Why? Gamespy builds a server list and then pings all of the servers in the list (1000s of game servers). By creating all these pings, it creates 1,000s of quick connections in a VERY short period of time. Until these sessions timeout via the IP MASQ timeouts, the MASQ tables become "FULL".
So what can you do about it? Realistically, don't use programs that do things like this. If you do get this error in your logs, find it and stop using it. If you really like GameSpy, just don't refresh the server too often. Regardless, once you stop running this MASQ'ed program, this MASQ error will go away as these connections will eventually timeout in the MASQ tables.