This is my favorite story about stray Civil War ordinance. It concerns a cannon that sits in the North Portico of the Rhode Island State House in Providence Rhode Island. Fortunately, the situation was resolved safely. The quotes are from a web site on the capital building.
"The gun was one of six used by the First Rhode Island Light Artillery, Battery B, at the Battle of Gettysburg. At around 1 p.m. on July 3, 1863, just before General George Pickett's assault on Union lines, two cannoneers were reloading this weapon when it was struck on the left side of the bore by a Confederate shell."
"The muzzle of the gun was altered by the heat of the explosion, and efforts to reload it with new shot were unsuccessful despite attempts to whack the ball into place with a hammer and axe. As the barrel cooled, it contracted, wedging the cannonball firmly inside, where it remains today."
"In 1874 the cannon was returned to the State of Rhode Island at the request of Battery B veterans."
"In 1962 someone realized that since the cannonball had never been removed from the muzzle of the gun, two-and-a-half pounds of black gunpowder must still be inside the barrel. Members of the Rhode Island National Guard's Army Corps of Engineers drilled holes in the barrel and submerged it in water so that the powder could be removed safely."
Link to full story:
http://www.quahog.org/attractions/index.php?id=62
Link to State House
http://www.woonsocket.org/river/statehouse.htm