Case example: while I don't remember all of the details (it was 15 years ago), there was an exhibit in a small museum in my hometown that had several civil war era percussion fuse cannon balls. An explosives-certified agent at the local ATF office heard that one of the curators would occasionally show children visiting the museum how neat they were, since they would warm up when you shake them. Ends up they were still live, but that the explosive had degraded; a little more energy (say, from dropping it after shaking it for a while) could have set the charge off, killing or seriously injuring the kids at the exhibit. They were safely destroyed, but it stuck in my mind as a cautionary tale not to screw around with old ordinance.