SQL> exec DBMS_APPLICATION_INFO.SET_CLIENT_INFO ('0');

PL/SQL procedure successfully completed.

SQL> CREATE OR REPLACE TRIGGER tr_tbl1
  2  BEFORE UPDATE ON tbl1 FOR EACH ROW
  3  begin
  4   DBMS_APPLICATION_INFO.SET_CLIENT_INFO (nvl(userenv('client_info'),'0')+1);
  5   dbms_output.put_line(userenv('client_info')||' '||:old.v||' '||:new.v);
  6  end;
  7  /

Trigger created.

SQL> merge
  2    into tbl1
  3    using tbl2
  4    on (tbl2.id = tbl1.id)
  5    when matched then
  6      update set v = tbl2.v
  7  /
1 1 1
2 1 2
3 1 3
4 1 1
5 1 2
6 1 3
  using tbl2
        *
ERROR at line 3:
ORA-30926: unable to get a stable set of rows in the source tables


SQL> select * from tbl2;

        ID V
---------- ----------
         1 1
         1 2
         1 3
         1 4
         1 5
         1 6
         1 7
         1 8
         1 9
         1 10

10 rows selected.