I know its legal, I know sky gods can pull it off BUT, the one that sets my teeth on edge is a right base join in a left hand circuit that is in use and is used pretty constantly by student pilots.
I have no problem with this join when the circuit is empty, in fact I have done it myself. However, I have experienced the gut wrenching feeling of a recently "gone solo" student, who has called position all of the way round the circuit as appropriate, at late downwind, hearing someone call "joining right base" (I was that student).
Probably made me a better pilot, for sure I listen to the radio, mentally picture what's where in the whole area and keep a very good look out. It was, however, a steep learning curve and sobering welcome to the big boys world of the circuit..