I have seen it a bunch of times here in Southern Maryland, a few times people have taken the cache home to investigate and then returned it nearby.
In a nearby puzzle cache, a muggle found the second stage and then decided to hide her own "tuperware" nearby. Since she had no idea what geocaching was and didn't have a GPSr, she left makeshift "Letterbox" style instructions to get to her treasure. After the owner finally found her cache he added it on to his puzzle as a bonus cache. Since then after leaving notes and such for her, the local caching community has gotten her to join the site and come to a few events.