In movies and film, the tendency of the hero's shots to hit more, and the enemy's to hit less. Summarized as:
1. The more outnumbered the heroes are, the more the enemies will miss.
2. The more impossible and desperate the shot, the more likely it will hit.
The stormtrooper effect in action:
Watch Star Wars IV: A New Hope. The stormtroopers really can't hit the broad side of a barn, can they?
Gimli in Lord of the Rings III: Return of the King:
"Certainty of death. Small chance of success. What are we waiting for?"