A query such as:

select (select e4 from pm1.g1 X_1), pm1.g1.e2 from pm1.g1, /*+ makeind */ (select e1 from pm2.g1) AS X1, (select distinct e2, e3 from pm3.g1) X2 where pm1.g1.e3 = X1.e1 and pm1.g1.e2 = year(X_2.e3)

Will fail to process correctly as the X_1 and X2 aliases are already in use, which will cause an AssertionError during processing attempting to reference X_1.e1.