Thanks it's making more sense however you said

"then in instance recovery Oracle reads Redo logs and RBS and noticed I had an uncommitted transaction so it rolls back my transaction (and hey the redo has been overwritten), so no big deal"

Where does it notice this from, if the log recording the transaction is overwritten how does it know that this is uncommitted?

Or is it just the fact that it has an Undo Segment and no entry in the current redo log that forces SMON (or SP) to roll it back.

Thanks again.

A