One of the basic tenets of gun safety is not to point a weapon at anything unless you want it destroyed. Given this a science show, I would expect them to have a grasp of the physics involved in stopping a bomb and should therefore ensured that everything in a possible target zone was clear and that their mechanisms to stop the ball would be adequate to stop it.
Mistake 1: Whoever their weapons expert is should be seriously questioned for not knowing how much recoil the cannon would have and how it would affect trajectory. I realize we don't use cannons much anymore, but this is basic physics. Actually the scientists on the show should be ashamed, too. I say this because they blamed the recoil, not an improperly functioning gun. If there was a flaw in the gun, it would still be a mistake in not properly examining the weapon, but I would find it more understandable than a basic physics error on a science show.
Mistake 2: Not have adequate barriers to stop the cannon. Due to science error above, they didn't have enough water tanks or position them properly, so there was only a brick wall which there weapons guy would presumably know was inadequate.
Mistake 3: Beyond the wall was a populated area. See basic gun safety.
Mitigating factor: Apparently, beyond the brick wall was a hill into which the cannonball was expected to bury itself? If that's the case - and assuming the hill was big enough - well that changes my opinion entirely. It completely wipes out mistake #2 and #3, leaving only mistake #1. And mistake #1 is forgivable as the hill should have provided adequate protection.
However, it seems like the hill wasn't tall enough to compensate for bad aim. Their bad. And really, this is basic science and basic gun safety. I don't understand why a well funded, knowledgeable group with their resources could make such basic mistakes.
Since no one got hurt, I hope they'll reevaluate their experts and precautionary measures but I don't find them totally reprehensible. Assuming the hill should have stopped the cannon, of course.