Inactive undo tablespace, still needed?

Question

Dear All,
I am using ORACLE 10.2 database server.
I changed the active undo space from USDOTBS1 to UNDOTBS2 and bring the inactive one to offline (normal mode).. The tablespace UNDOTBS1 is empty. As I read in the documentation an undo table space cannot be put offline if it still contains any data used by ongoing transactions;
However, when some DML statements (completely new transaction) executes, Oracle complains about having no access to the UNDOTBS1, and datafile associated with it. I don’t understand how it is possible since the active undo tables space is UNDOTBS2.
After bringing UNDOTBS1 back online (without making it active), everything works fine.
The error is as follows:

Answer 1

So how did you confirm there were no outstanding snapshot dependant transactions against the undo tablespace?
http://stanford.edu/dept/itss/docs/oracle/10g/server.101/b10739/undo.htm
Looking at the section on dropping undo talks about dependant transactions on before snapshots of undo.
"However, since DROP TABLESPACE drops an undo tablespace even if it contains unexpired undo information (within retention period), you must be careful not to drop an undo tablespace if undo information is needed by some existing queries."