By reading the title of this post, you probably have a general idea of what I am talking about. Either A, people lagging in online games avoiding being killed, or B, games that generally are built badly, and count impacts to the side of someones head as a direct hit.
First, the topic of people lagging. Now, most servers for online games have lag compensation, where they "estimate" where the player is and make it appear to everyone else that the person is there, simply because the connection between the lagging player and the server is weak. To the lagging player, who may or may not be having choppy gameplay, he still has a human mind, and may make sudden movements that the server cannot keep up with on a slow connection. So although it may appear that the player is in front of you firing in the opposite direction, he may actually be behind you, putting a sharp object into your spine. However for someone to move that great of a distance secretly through lag is very rare, as servers often take precautions to prevent this, such as having a minimum connection speed to the server. This also brings up the topic of lag switches, which temporarily disables or severely weakens the players internet on purpose on a flick of a switch, allowing them to take advantage of this lag. Obviously most servers thesedays have counter measures against this, as I mentioned previously.
The second topic, bad hit boxes, is simply the fault of the game, and is rarely improved over DLC. These days it is pretty accurate, you can shoot someones ear and it will kill them, but shooting just outside their ear wont. However slightly older games or new games with older engines still suffer from this, and it is up to the developers to ensure this is avoided. Often times it is only noticed in first person shooters, as sniping often has to be accurate down to the skin cell for it to count.
Why should this matter? Just think about what could have been at fault before calling "HAX!"