It happens in the US too, occasionally. The campus of my alma mater, along with a good chunk of the surrounding neighborhood, was taken over by the U.S. Army during both World Wars and used as an artillery range and, during World War I, a chemical weapons development center. (See: http://en.wikipedia.org/wiki/Camp_American_University)

As a result, even up to the mid-1990s, when I was there, it was not unusual to see a block or two closed off periodically because someone digging had found unexploded ordnance there. And due to the chemical weapons work, these discoveries were more disconcerting than usual, because who knew if it was just a standard high-explosive shell or something loaded with mustard gas, or worse?