https://secure.wikimedia.org/wikipedia/en/wiki/Checksum

- The master server has a saved checksum of "semi.png".
- Whenever you start the game it creates a checksum (MD5, SHA or whatever) on your local copy of "semi.png".
- If the checksums are different you won't be able to play.

Here some checksums for "semi.png" (maybe he used CRC which could be exploitable):

CRC32: 33CBE54D
MD5: 399D837E8EEAC84CE35931DB621D2971
SHA-1: AA5EC2389674F5274BF4C4D0306F3F358EAB03CB
SHA-256: DA6B629DB4A26FCE007E765609654F4FF6B61486976C2F079B062BC9B911B8C5

That sounds about right. :(

Just searched the changelog.

"-ironsights now checksummed"

Don't know what that means exactly, but I'll take a guess: If the PNG's numbers don't match up to the ones set, it'll fire up an anti-cheat function.

Anyone want to cut in for my stupidity, please?

That's what I was thinking. But what I don't get is how would the client know?

I think Ben inserted something to prevent changing the ironsights, as some could be completely invisible save for a small crosshair (taking away most of the obstruction).

Recently I decided to install some skins and they look awesome. I then changed my crosshair because I was getting annoyed of the standard one. I also realised that the iron sights did not match my MP5 at all. So I had a go at changing them as well. I tried to do it myself but when I replaced the original .png, when I was alive, and only when I was alive, all of a sudden my screen would act as if I was looking up then looking down super fast, but grenades and bullets would act as if I was perfectly still. My question is this. Is this supposed to happen? Is it a bug, or something Ben put in to keep gameplay fair?