the old devices were based on heart (cardiac) pacemakers which were designed to turn off w/ a magnet. A bit of a dark side to the old chestnut about it not being a bug, it's a feature...

The newer ones use bluetooth so hopefully less susceptible. But yeah, have had case reports of refridgerator doors, mall security, TSA agents overzealous w/ their wands, and one instance w/ a Prius, etc. There is a gizmo they get that they can put up to their device and turn it back on; that being said, a lot of ppl have lost theirs, or have let the AAA batteries in them run out...