Not an excuse, because they shouldn't have a finger on the trigger when activating the light, but this does point out the validity of the argument that the rail on a pistol is not the appropriate place to be placing a flashlight, because it violates a fundamental rule of gun safety: "Never point the muzzle of a firearm at anything you are not willing to destroy."
Of course, weapon light proponents counter that the lights are strong enough that the bounce provides adequate illumination so the muzzle doesn't have to point directly at a suspect (or innocent bystander). The problem is that police aren't taught to perform clearing exercises by sweeping the room with the muzzle pointed at the floor ...