The invention of mass produced explosive shells, in the 19th century, led to a lot more of these shells being used. But this left thousands of these shells buried in the ground. Users of these shells were only vaguely aware of how many of them were duds. But over the last century, better quality control and testing has revealed that the dud rate could be quite high. In the 19th century, the dud rate was probably 10-20 percent, mainly because of the crude fuze technology available. The "Parrot" type shells were most typical of the American Civil War period. These were commonly about 150mm caliber, and carried a few pounds of black powder. When these shells exploded, they dispersed both metal fragments, and metal balls designed to maximize damage to people.
As was recently demonstrated, that black powder could retain some of it's explosive power after nearly 150 years. More recent duds are much more lethal. This is partly due to higher manufacturing standards, more powerful explosives, and larger shells. While Parrot shells were usually less than a foot longer, today's 155mm shells (the most common type), are longer, heavier (up to 100 pounds) and carry far more explosive. A lot more of these modern 155mm/152mm/150mm shells have been fired in the last century. We're talking over 100 million shells, at least, and over five percent of them were probably duds. Many of those millions of shells are still in the ground, in battlefields from northern France, to several parts of Russia, the Iran-Iraq border, the 38th parallel in Korea, northern China and many islands in the Pacific. People will be killed by these "dud" shells well into the next century. So beware of duds, no matter how old they are.
The casualty count from the American Civil War just increased by two. Two American Civil War collectors were recently injured when an 1860s era explosive shell, did just that as they were trying to remove the explosives. This is one of the oldest dud shells to injure someone, in this case nearly 150 years after the shell was manufactured.