One of the most common mistakes in battlefield care is attempting to treat the casualty before the threat is eliminated.
It is often quoted that on the battlefield the best medicine is fire superiority. RETURN FIRE. ELIMINATE THE THREAT! Then deal with the casualty.
Lack of familiarity with the equipment available.
Lack of practice with techniques under battlefield conditions.
Lack of practice with equipment under battlefield conditions.
The vast majority of these mistakes can be eliminated or reduced through effective training and practice.