SQL> CREATE TABLE test_tab(a NUMBER(1), b NUMBER(1));

Table created.

SQL> INSERT INTO test_tab VALUES (1,1);

1 row created.

SQL>
SQL> CREATE OR REPLACE PROCEDURE test_prod AS
  2    v_a NUMBER;
  3    v_b NUMBER;
  4  BEGIN
  5
  6  SELECT a,b
  7    INTO v_a, v_b
  8    FROM test_tab
  9   GROUP BY a;
 10
 11  END;
 12  /

Procedure created.

SQL>
SQL> BEGIN
  2  test_prod;
  3  END;
  4  /
BEGIN
*
ERROR at line 1:
ORA-00979: not a GROUP BY expression
ORA-06512: at "PFK.TEST_PROD", line 6
ORA-06512: at line 2

SQL> CREATE OR REPLACE PROCEDURE test_prod AS
  2    v_a NUMBER;
  3    v_b NUMBER;
  4    b number;
  5  BEGIN
  6    SELECT a,test_prod.b
  7    INTO v_a, v_b
  8    FROM test_tab
  9    GROUP BY a;
 10  END;
 11  /

Procedure created.

SQL> exec test_prod;

PL/SQL procedure successfully completed.

SQL> CREATE OR REPLACE PROCEDURE test_prod AS
  2    v_a NUMBER;
  3    v_b NUMBER;
  4    c number;
  5  BEGIN
  6    SELECT a,c
  7    INTO v_a, v_b
  8    FROM test_tab
  9    GROUP BY a;
 10  END;
 11  /

Procedure created.

SQL> exec test_prod;

PL/SQL procedure successfully completed.

SQL> CREATE OR REPLACE PROCEDURE test_prod AS
  2    v_a NUMBER;
  3    v_b NUMBER;
  4    b number;
  5  BEGIN
  6    SELECT a,b
  7    INTO v_a, v_b
  8    FROM (select a, b c from test_tab)
  9    GROUP BY a;
 10  END;
 11  /

Procedure created.

SQL> exec test_prod;

PL/SQL procedure successfully completed.

SQL> CREATE OR REPLACE PROCEDURE test_prod AS
  2        v_a NUMBER;
  3        v_b NUMBER;
  4      BEGIN
  5      SELECT a,b
  6        INTO v_a, v_b
  7        FROM test_tab
  8       GROUP BY a;
  9     END;
 10  /

Procedure created.

SQL> CREATE OR REPLACE PROCEDURE test_prod AS
  2        v_a NUMBER;
  3        v_b NUMBER;
  4      BEGIN
  5       SELECT  k,s
  6         INTO v_a, v_b
  7   FROM test_tab
  8  GROUP BY a;
  9      END;
 10   /

Warning: Procedure created with compilation errors.

SQL> sho err
Errors for PROCEDURE TEST_PROD:

LINE/COL ERROR
-------- -----------------------------------------------------------------
5/6      PL/SQL: SQL Statement ignored
5/16     PL/SQL: ORA-00904: "S": invalid identifier
SQL> 
SQL> desc test_tab
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 A                                                  NUMBER(1)
 B                                                  NUMBER(1)

SQL> select banner from v$version;

BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Prod
PL/SQL Release 10.2.0.1.0 - Production
CORE    10.2.0.1.0      Production
TNS for 32-bit Windows: Version 10.2.0.1.0 - Production
NLSRTL Version 10.2.0.1.0 - Production

SQL>  CREATE OR REPLACE PROCEDURE test_prod AS
  2         v_a NUMBER;
  3         v_b NUMBER;
  4       BEGIN
  5        SELECT  a,test_tab.b
  6          INTO v_a, v_b
  7    FROM test_tab
  8   GROUP BY a;
  9       END;
 10   /

Procedure created.

SQL> exec test_prod;
BEGIN test_prod; END;

*
ERROR at line 1:
ORA-00979: not a GROUP BY expression
ORA-06512: at "SCOTT.TEST_PROD", line 5
ORA-06512: at line 1