with t as (
           select 1 id,-1 val from dual union all
           select 2,2 from dual union all
           select 3,-3 from dual union all
           select 4,0 from dual
          )
select  id,
        val,
        case val
          when 0 then 0
          else
            ln(abs(val))
        end x
  from t
  order by id
/

        ID        VAL          X
---------- ---------- ----------
         1         -1          0
         2          2 .693147181
         3         -3 1.09861229
         4          0          0

SQL> 

with t as (
           select 1 id,-1 val from dual union all
           select 2,2 from dual union all
           select 3,-3 from dual union all
           select 4,0 from dual
          )
select  id,
        val,
        case val
          when 0 then 0
          else
            sum(ln(abs(val))) over(order by id)
        end x
  from t
  order by id
/
ERROR:
ORA-01428: argument '0' is out of range



no rows selected

SQL> 

orclz>
orclz> select * from scott.dept where deptno/deptno=1 or deptno/0=1;

    DEPTNO DNAME          LOC
---------- -------------- -------------
        10 ACCOUNTING     NEW YORK
        20 RESEARCH       DALLAS
        30 SALES          CHICAGO
        40 OPERATIONS     BOSTON

orclz> select * from scott.dept where deptno/0=1 or deptno/deptno=1;
select * from scott.dept where deptno/0=1 or deptno/deptno=1
                                     *
ERROR at line 1:
ORA-01476: divisor is equal to zero


orclz> select * from v$version;

BANNER                                                                               CON_ID
-------------------------------------------------------------------------------- ----------
Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production              0
PL/SQL Release 12.1.0.1.0 - Production                                                    0
CORE    12.1.0.1.0      Production                                                                0
TNS for 64-bit Windows: Version 12.1.0.1.0 - Production                                   0
NLSRTL Version 12.1.0.1.0 - Production                                                    0

orclz>

SELECT  CASE y
          WHEN 0 THEN 0
          ELSE x / y
        END
  FROM  TBL

 SELECT  x / y
   FROM  TBL
   WHERE y != 0
      or y IS NULL
UNION ALL
 SELECT  0
   FROM  TBL
   WHERE y = 0

SELECT  CASE y
          WHEN 0 THEN 0
          ELSE x / y
        END
  FROM  TBL

select *
from tbl
where c1 > 0
and funcx(c1) = 1
/

funcx (p1 in number) return number is
begin
   return (1/p1);
end;
/

select *
from tbl
where c1 > 0
and funcx(c1) = 1
/

1. where c1 > 0
2. and funcx(c1) = 1

Are you soft parsing or hard parsing?
Is the system busy so you will be downgraded to serial execution or will you be allowed to exploit PQ this time?
Did you recollect stats last night?
Did Oracle decide to use dynamic sampling this time around?
Did cardinality feedback from the last execution change the optimizer's idea of a good plan?

where c1 > 0
and funcx(c1) = 1