As you know, packets have a payload descriptor so you can check to see what kind of package we are dealing with. So when I encounter a packet with an invalid descriptor (other than 0x00, 0x01, 0x80, 0x81 or 0x40) I assume for one reason or another the stream lost sync and I drop the connection. However, This occurs quite a lot, and I don't actually have the feeling that I'm loosing sync, but I think it are custom codes from other clients.
Could this be? If so, can I get a list of these custom codes (the known ones) somewhere so I can ignore the packet if it's one of these, and drop the connection if it is still another.

The only ones I believe might be used are 0x02 and 0x03, but those were only proposed and shouldn't actually be used. But when these "invalid" messages appear, what I would do is assume the payload is correct, read it but then drop it from further distribution. I'd also keep a counter for the occurence - if it happens too often within a short timespan, I'd consider the other client to be faulty and drop the connection.

30 is used for query routing

Thanx, I think I'll go for the first one, to ust ignore a number of successive invalid packets, before dropping the connection.