Hmm,
I've seen where the little (14ga ?) Cable support wire was carrying all the neutral current. It would sometimes smoke depending on what they turned on in the house (balance) but other times it would be Ok.
In your situation the waterpipe could still have been connected somewhat through the Furnace being grounded or something like that. There's also Cable and Telephone connections that may have helped. I say this because the earth alone is just not that conductive by itself. Unless loads were always nearly perfectly balanced they should've noticed something was up.
Bill
