more vs. larger rollbacks

I know the rule of thumb is, lots of connections and users, OLTP, lots of little and
medium sized rollback segments, big batch jobs, fewer, and larger rollback segments.

But..

We have someone doing some preliminary datawarehouse loads, and have provided
3 1.5G rollback segments, combined equalling 4.5G which is bigger than the database
itself (I know that's ridiculous) , but our user is getting rollback too small, and the high
water marks on these three 1.5G rollback segments never get above 300M. Still
PLENTY of room, at far as I know.

So..
I'm guessing that there are certain situations in Oracle, even with one user and one session,
where Oracle will ask for another rollback segment, rather than extend one where there
is plenty of room. Can anyone either tell me I'm wrong, or tell me what kind of circumstances would cause this?

As to the errors, I have not seen specifically what it is. User says it's rollback too small,
but for all I know it could be need more rollbacks or snapshot too old. Regardless..any
feedback would be appreciated.

• Dc.