ERROR: btree: lost page

Hi,
i have a huge DB on the web and manage it with some perl scripts.
recently i underestood that some fields of one of th tables make the
postgres not to work proparely.

i found out that in the table X, and row for user Y, the field Z have
problem. when i type : "update X set Z='something' where user='Y' ; " in
psql intractive environment, this error message occures :

"FATAL 1:
btree: lost page in the chain of duplicates"

then the psql don't accept any request and queries!!!

when i delete this user from table X, and re-add him with a new user name,
the problem doesn't appear. also i can't add a user with his last user
naem. !!!

i am really confused . may be this is a bug of postgres?!

plz send ur comments to me

sincerely
mehdi roomi
roomi(at)safineh(dot)net