There are also many types of aim-assist hacks for first person shooters that also work by hooking into graphics library calls. OGC Hook for Counter-Strike / HL1 had a multitude of options beyond making walls transparent.
If the location info for enemies is at some point passed to a lower level library which has widely available code, those calls can be hooked into and the location, scale, rotation, etc. data can be used to precisely calculate the best vector for firing upon that enemy.
Anti-cheat systems like PunkBuster often include heuristic-type detection (how often does this player get a perfect shot, how often do they spin around 180 degress to land a one-shot kill, etc.) in addition to memory and process monitoring.
Ultimately the task of preventing cheating in multiplayer games is a game of cat and mouse. As new cheats are developed, new anti-cheat methods are created, and then new cheats are built to avoid the new anti-cheat technologies.
As Tetrad said in his response, the only hard and fast rules for "game security" are to provide the client as little information as possible, and to trust the client as little as possible. Different types of games will have different requirements that can impact how well you can adhere to these rules, though.
In an FPS game, because the speed of gameplay is so fast, it is often not possible to only provide the client with data it should know about this exact instant. If you don't tell the client there is an enemy around the corner and the player does go around that corner you suddenly have to send them that information in a matter of milliseconds or risk having the player be killed by an enemy they didn't ever see.
The only "fix" for these issues that is currently available would probably be OnLive. Because that service only delivers audio and video content from a client running safely in the datacenter the only means for cheating would be to essentially create a human AI that could analyse the incoming video stream and take action based upon that -- a task that few, if any, cheaters are up to.