One of the most difficult things troops in combat have to learn is how to deal with the chaos. In Iraq, for example, there are a lot of firefights, many unplanned. These often involve non-combat troops, usually those in a vehicle stopped by a roadside bomb or ambush. Unlike the infantry, these troops have not had the specialized, and intense, training that prepares them for these gun battles. They don't know the moves, or what to expect. These troops quickly realize that, under fire, its very difficult to coordinate the actions of several people armed with rifles or light machine-guns. One group of reservists made a list of the reasons they encountered for why one of their buddies was not firing where he (or she) was supposed to during those firefights. 

It was because the other soldier was; wounded, dead, pinned down by enemy fire, didnt understand the last command they had received, was shooting in the wrong direction (for any number of reasons), was in the wrong place (for any number of reasons), was blinded by smoke or debris (from an explosion or sand kicked up by bullets), was moving (for any number of reasons), was telling someone else what to do, was changing the barrel in a machine-gun (have to do that when the barrel overheats), was trying to get help via the radio, their weapon was jammed (a round had not loaded properly), was reloading, was adjusting their gun sight, was distracted (for any number of reasons), had panicked (for any number of reasons), was trying to listen for commands, had a damaged weapon, had sights that were not adjusted properly, battery just went out on a night sight (at night), or dazzled by light (for any number of reasons) during a night fight. The reservists were amazed at how long this list got, after just asking a dozen or so people in the unit who had been caught in a firefight. Everyone wants to hang out more with infantry guys and, you know, just talk.