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ORA-01008: not all variables bound

The following SQL statement is parametrized with two bind variables, but only one bind variable is bound. Hence, Oracle throws ORA-01008: not all variables bound.
declare
  cnt number;
begin
   execute immediate
      'select
          count(*)
       from
          dual
       where
          1 = :a and
          2 = :b'
   into cnt
   using 1;
end;
/
In SQL*Plus, this error can also be forced like so
SQL> set define off
SQL> var res varchar2
SQL> exec execute immediate 'select dummy from dual where 1 = &num' into :res;

See also

Other error messages such as

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