You most likely have seen this error before:
ORA-01719: outer join operator (+) not allowed in operand of OR or IN
Cause: An outer join appears in an or clause.
Action: If A and B are predicates, to get the effect of (A(+) or B), try (select where (A(+) and not B)) union all (select where (B)).
In 11gR2 it is no longer raised for predicates like column(+) IN (constant1, constant2, …).