Search notes:

ORA-30008: CONNECT BY clause must reference a PRIOR

select
   level
from
   dual connect by level <= 5;
--
--      LEVEL
-- ----------
--          1
--          2
--          3
--          4
--          5

Using connect by without prior clause is not allowed, but not enforced by default.

The following «fix» enforces it:

alter session set "_fix_control" = '6520717:on';

Now, the same statement throws ORA-30008: CONNECT BY clause must reference a PRIOR:

select
   level
from
   dual connect by level <= 5;

ORA-30008: CONNECT BY clause must reference a PRIOR

See also