I agree with Al, trigger control -- mostly snatching or jerking.
The second most common problem is gripping the gun too tight.
That's always an interesting discussion.
Mas Ayoob teaches "crush" grip and shoots with it very successfully.
My take is that, during a scary event such as lethal encounter, you're going to tense up and be gripping the gun hard as an automatic, uncontrolled reaction, anyway.
Learning to shoot that way might make sense.
If you're already gripping hard, you can't squeeze and jerk the gun when you pull the trigger, as Ayoob points out.
Don't know if any data exitst from real shooting re: grip. Those involved probably wouldn't be aware of it.