unused columns in version 11.2.0.4

Hello

I have not done much work with unusable columns so I am trying to test is before implementing in production environment

Here is my test

select * from test4;

      COL1       COL2

---------- ----------

         1          1

         1          1

         1          1

         2          3

alter table test4 set unused (col2);

Table altered.

select * from test4;

      COL1

----------

         1

         1

         1

         2

alter table test4 add col2 number;

Table altered.

select * from test4;

      COL1       COL2

---------- ----------

         1

         1

         1

         2

alter table test4 drop unused columns;

Table altered.

select * from test4;

      COL1       COL2

---------- ----------

         1

         1

         1

         2

For some reason I was under impression that once I have made a column unused I can not add it to the table. Is that an incorrect assumption (based on my quick test) or is my test flawed?