SQL & PL/SQL :: ORA-00979 - Not A GROUP BY Expression At Runtime?

I ran into something that has me wondering about which errors are caught at compile-time, and which are caught at runtime. There is a pretty big package with lots of complicated queries that heavily use GROUP BY, and in the last couple of years I thought I never had the case of an ORA-00979 happening an RUNTIME, which is now happening.

The last heavy changes in that package did I make in the Oracle 9.2 days, and I *believe* (although my memory might be wrong) that back then the ORA-00979 errors came up at compile time.

So is it normal that the error doesn't show up during compile time, only during runtime, or should it be considered a bug?

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