So a while back, I upgraded my entire video game-internet connection system, with a new 1 gigabit per second router and upgrading to the new "RoadRunner Turbo" Cable, which is around 20 mbps I believe.  Since then I've been playing a lot of Call of Duty, first 4, now WaW, and I can't help but notice that many enemies seem to see me around corners before I see them.  Now, I've been gaming for around 14 years (I'm 16), and I should think that my reflexes are above par at this point.  The logical conclusion I keep coming to as to why dudes can shoot me as I'm rounding corners (without UAV/Recon Plane, with Dead Silence a.k.a They don't know I'm coming) is lag. 
     Call of Duty has been especially helpful in making me realize this through their Killcams, where what I see the enemy saw never matches up with what I saw.  For example, the most common thing I see in the kill cam is the enemy rounding a corner, spotting me, and opening fire.  My "good connection" view of this is seeing the enemy round a corner and immediately opening fire on him, at which point I die within 2 shots, because apparently he's already seen me and taken aim.  It is my opinion that my good connection lets all enemies see me in real time, while I see them anywhere from 1/10th to 1 whole second later than what's really happening. 
     Another common happening is being in an open firefight with someone, shooting at the dead center of their chest while they move.  Because of this half a second or so of delay, of course my shots are well behind them, yet theirs are right on target, because I am where they see me as being, yet they are not. 
     Lastly, we come to knives.  It is impossible, yet for the faintest chance, for someone with a decent connection to get a knife kill again someone without.  For one, when attempting to charge them with a knife, the person with the good connection will undoubtedly be off target as shown on the Killcam, even though they were dead-on by their view.  On the Killcam, it almost seems laughable how off they will be, charging at thin air with knife in hand and then the lagger lazily shooting them in the back.  On the defensive side, the opponent is likely to charge you, what do you do?  If you attempt to swipe the knife where you are, he will win because he is in the charging animation.  If you attempt to move forward to charge, you're dead before you swing the knife.
     Basically it boils down to this.  Lag is an inevitability in today's gaming world.  There's no way to ever be rid of it, and those who lag have no reason to get good connections, because they benefit from their own lag (hence devices such as the "lag switch", by which a player can literally generate inordinate amounts of lag for themself in an otherwise lag free environment).  Targeting systems deal with where players are then and there, not where they appear to be.  Ultimately, in an attempt to play competitive games, those who lag can see where those who don't are, and those who attempt to improve their connection will find that they are much easier targets.