[gman3]

Here are some:

Not watching the hands;
Taking a bad position;
Not maintaining your weapon in a serviceable condition;
Not maintaining proficiency with your weapon;
Not maintaining situational awareness, ignoring danger signs;
Not maintaining proper physical condition;
Tombstone courage;
Not understanding your own limitations;
Bad Attitude/No combat mindset;
Relaxing too soon;

These will get you killed much faster than any considerations of timers, reloading magazines, putting brass in your pocket, etc.