Always read the comments. While reading slashdot's take on the "home-built mortar" story we linked up last week (HA!), I found this old, but good, story about a discovery almost exactly one year ago in the Chicago Field Museum's collections:

A Museum in Chicago has found some of its antique firearms were loaded and could have gone off at any time.

This sort of thing actually happens a lot more often than you'd think. Two incidents from air museums: Some time in the early 90s janitors were surprised at the smell of kerosene while cleaning one of the Air Force Museum's displays. Jet fuel was pouring from the B-58 Hustler. Turned out the thing had been parked in the museum building about 1/2 full of gas. Nobody'd ever bothered to check, until finally one of the tanks corroded through.

While pulling out the pieces of an FW-190 (German WWII figher plane) from storage for restoration in the mid 80s, the Smithsonian Air & Space Museum staff heard a strange sloshing coming from the fuselage. On opening up the gas cap, they found the tank to be about 1/5th full. Turned out to be a valuable find, because this was a late-war aircraft that was using some weird coal-derived fuel because the Nazi's couldn't refine avgas. Nobody'd actually seen an example of the stuff until that point. The plane had been disassembled in one of their storage hangers for more than thirty years.