In both cases, the passengers had no intent to harm, in the case of the cannonball it had been collected as a souvenir on a diving trip, but of course these events have the potential to cause serious harm. Cannonballs found at the bottom of the ocean can still retain their explosive elements and have been known to detonate.
Other recent reports of dangerous items collected by TSA screeners include a cattle prod, loaded guns, and even a lipstick knife.
Attempting to board a plane with these items can have some unfortunate side effects. The less serious side effect is the potential for delaying flights, which can have a ripple effect throughout the airline flight plans for that day. Another side effect is that passengers who attempt to board with these items are often denied boarding. In all cases, these items are confiscated by the TSA, and in the case of loaded guns, the passenger is in serious trouble.
So, should you find a hand grenade or other interesting artifacts on your trip, consider leave them where you found them and settling for a photograph instead.