Hello,

This is the complete explanation available in METALINK :

**********************
Error: ORA 1594
Text: attempt to wrap into rollback segment (%s) extent (%s) which is being fr
eed
-------------------------------------------------------------------------------
Cause: Undo generated to free a rollback segment extent is attempting
to write into the same extent due to small extents and/or too
many extents to free
Action: The rollback segment shrinking will be rollbacked by the system;
increase the optimal size of the rollback segment.


*** Important: The notes below are for experienced users - See [NOTE:22080.1]


Explanation:
1594 is reported when a rollback segment is being shrunk but there is
not enough space in the current undo extent to complete the job.
Shrinking uses whatever is available in the current extent in the
rollback segment. If no more space is left and the extent you
would move into is one that is being freed you will get 1594.
This error is harmless. When you retry, chances are you will not get
the same error again.

Usually this is caused by running a transaction after a huge
transaction commits. Because of the setting of OPTIMAL, a very large
number of extents need to be deallocated all at once.

One way to avoid this problem is to have larger extent size.
Another way is to have a larger OPTIMAL setting, so that only a smaller
number of extents will be removed at one time.

SET TRANSACTION USE ROLLBACK SEGMENT is also a good idea for large
jobs so you know which rollback segment they will use.


Note: There are changes to the way SMON shrinks rollback segments
in 7.2. It should try to shrink a small number of extents at
a time.


Hope this helps

Regards