(come to think of it, would anyone take the cannonball seriously?)
I think it was about a year ago when somebody left a box clearly labeled "verdacht pakketje" ("suspicious packet") at Utrecht Central Station, which is the main train hub in the Netherlands. I think it had a mechanical (ticking) alarm clock inside as well. This was clearly intended as a prank, not to be taken seriously.
This brought train traffic throughout the Netherlands to a standstill, because the police did take it seriously, stopped all train traffic going through the station (which had a knock-on effect throughout the country), evacuated the station and spent three to four hours investigating.
A lot of people were late for dinner that night.
So a large cannonball shaped object with an actual fizzing fuse? I don't think they're going to take any chances.