Developers place too much trust in everything, they assume that certain data cannot be faked and therefore these pieces of data can be used as a Trojan horse. Lets take the REMOTE IP of a user, it seems a trusted source because of the TCP/IP connection between the user and the server.

He points out the difference between HTTP_X_FORWARDED_FOR and REMOTE_ADDR and how, despite them being the same almost all of the time, shouldn't be trusted since they could be spoofed. He even includes an example script showing how it could be done (and how a bit of Javascript can even be inserted).