ORA-00905: missing keyword

In the following SQL statement, the parser expects the keyword end after 'n/a' but does not encounter it, so in Oracle releases prior to 23c, the error message ORA-00905: missing keyword is thrown.

select
   case when owner in ('SYS', 'SYSTEM') then 'n/a'
from
   dba_tables;

In 23c or later, Oracle specifed the expected keyword, for example ORA-02000: missing END keyword.

select
   col_1
into
   var_1
from
   tab_A;

select
    *
from
   dual
pivot(max(dummy));