Hi Mike,
The following links illustrate some random examples of incorrect counties, even though they do not refer to the Durham/Cumberland examples I referred to in my initial post:
http://search.ancestry.co.uk/cgi-bin/sse.dll?indiv=1&db=...(NEWCASTLE UPON TYNE WAS IN NORTHUMBERLAND, NOT CUMBERLAND)
http://search.ancestry.co.uk/cgi-bin/sse.dll?indiv=1&db=...(MIDDLESBROUGH WAS IN YORKSHIRE NORTH RIDING, NOT EAST RIDING)
http://search.ancestry.co.uk/cgi-bin/sse.dll?indiv=1&db=...(NEWCASTLE UPON TYNE WAS IN NORTHUMBERLAND, NOT WESTMORLAND)
These are just three random examples generated by doing a search of one of the surnames in my family tree. They aren't in fact my relatives but could be misleading for those searching for them by county, and I have come across countless examples in my searching. I am not certain whether the problem is confined to the north east of England.
Hope this helps.