Few options

Data corruption:

By analysing that table structure you can find the corrupted data

Block corruption:

DB_VERIFY utility
[url]http://technet.oracle.com/docs/products/oracle8i/doc_library/817_doc/server.817/a76955/ch09.htm#1936[/url]
DBMS_REPAIR utility
[url]http://technet.oracle.com/docs/products/oracle8i/doc_library/817_doc/server.817/a76956/repair.htm#8282[/url]

Hope this helps...