question about PANIC: hash table "Shared Buffer Lookup Table" corrupted

hi

I use Postgresql 7.4.19 wiht 8G memory on Redhat5 x86_64.
And I modify the postgresql.conf file,
when I set
shared_buffers = 786432                # min 16, at least
max_connections*2, 8KB each
(6G)
and import data.
[ PANIC:  hash table "Shared Buffer Lookup Table" corrupted ]
occur.

I set the shared_buffers to
shared_buffers = 393216  (3G)
no error occur.

Thanks for any idea.