Leaping on that bandwagon, you most definitely cannot use application contexts:-

Code:
SQL> SELECT SYS_CONTEXT( 'app_ctx', 'attr1' ) AS get_1
  2  ,      SYS_CONTEXT( 'app_ctx', 'attr2' ) AS get_2
  3  FROM  (SELECT CASE
  4                  WHEN 1 = 1 THEN
  5                    set_ctx.set_attr ('definitely', 'not')
  6                  END
  7         FROM   dual
  8         ORDER BY 1);

GET_1      GET_2
---------- ----------
definitely not