I think I can clear this up...
I was playing today and I joined a server that said ESFCS, and I joined and it said in the message this is a CS Based ESF server, and then I continued to be kicked by a wrong client.dll file.
Joined "CounterStrike" and I was told in the message this server has esf installed on CS and then I was continued to be booted by a client.dll error.
I'm thinking that he is joining HL Based installations of ESF, and that he is being kicked because the CS version of client.dll is different from the HL version.
Best bet mate - Get HL!