Recovery Manager (RMAN) problem

Hello,

I have the following problem. I need to implement a mechanism which
will restore a database to its state in the past. The restore
operation should start on demand. The machanism is supposed to be some
kind of antidote for user errors (e.g. a user had changed data in DB
and after that it turned out that all changes made were  wrong and DB
has to be restored to its state before the user changes).

For implementastion such mechanism I plan to use RMAN. I'm doing test
for the moment and I found out that it doesn't work as I thought. I do
the following:
1. I backup (with RMAN) a TABLESPACE on which database stores user
data
2. I make changes in a table (the table is stored on backuped
TABLESPACE)
3. I restore the TABLESPACE from the file backuped at point 1.

The problem is that changes made (2) after I backup a tablespace (1)
exists in the table (but I think it shouldn't because I restore the
tablespace)(3).
Could you please tell me what is the reason for that?
I appreciate any help.

Best regards,
krys_tek