When you change the TCP packet, intercept the packet, or intercept and change the packet it affects how the server receives them. If they aren't sent in the right sequence then the server will DC you. If you change the packet then the size of the packet itself is different than what is read in the header. If the size doesn't match what's in the header, then it will also DC you. TCP is a reliable protocol. You can do what you want with UDP as there are no checks done, but it is a lot more risky and easier to hack.