The discovery of a mummified cat in an early 19th-century Edinburgh building reminded me of one of the niftiest things in historical archaeology, magical deposits. Common caches hidden under floorboards or within walls include shoes, cats, or "witch bottles" specially prepared with pins and urine. These were magical charms in British culture, and still hold some power. As noted on Brian Hoggard's page on these charms and other folk magic, cats were often destroyed or otherwise noted by construction teams that discovered them. They can be creepy whether interpreted by the discoverer as an unfortunate accident or as an occult artifact, and in some cases are burned to cleanse the deposit and perhaps help the cat in the afterlife.
Update: First witch bottle still sealed (and presumably containing urine) found in Greenwich.
Update: Article from March 2008 has nice images and discussion
Update April 2009: Sealed shoes from Nova Scotia
Update June 2009: The sealed witch bottle has been analyzed