The ballistics of a cannon are well understood. That makes the potential trajectories of the projectile quite predictable. Someone chose to try to manage the trajectory instead of finding a location where every trajectory would be safe for bystanders.
In the context of filming a television program, that seems like a big mistake.
If the operators of a military firing range got that wrong, then I find it hard to believe that anybody could have realistically been expected to get it right.
"Bystanders" is probably the wrong word. I meant people not participating in the filming.
The failure in the stunt was that the cannon fired at an unexpected angle and the ball did not embed in the berm as they expected. If they had chosen a location where a ball fired at maximum velocity and at the worst case angle could not hit a person, that failure would have any negative consequences. So (I think) it was reckless to try to manage the trajectories at a less safe location.
In the context of filming a television program, that seems like a big mistake.