One of my friends works in a courthouse in a small town in Michigan. A few weeks ago, some guy brought in a package to the police station (same building) and goes 'I just inherited a house and found this in the attic, I don't want it any more'. He then left it on the counter and left. It turned out to be a WWII mortar shell that was rusting through. They (eventually) got the bomb squad there and took it out to a field to detonate.

You can find all kinds of dangerous things in old houses.