http://www.theregister.co.uk/2008/11/14/spectrum_map/

"Anyone hoping to use GPS around Meadowhall, a shopping complex near Sheffield, should be aware of an unusually high spike in the frequencies used by the satellite location service. This could be attributed to a welder or similar sparking equipment, but only more testing will determine if it's really an attempt by the owners of Meadowhall to ensure people stop to ask for directions."

From a non-geocaching perspective, this is even better:

"Even more bizarre is the 1.075GHz spike in the middle of Millom in Cumbria. This emanates from the square in the centre of the North-eastern town, without propagating down nearby streets, indicating a vertical transmission, but where it's coming from is anyone's guess."

Phil