Say I control a Sphinx of the Second Sun. At the beginning of my post-combat main phase an additional beginning phase is added after that main phase. Now I activate Aggravated Assault during my post-combat main phase and get an additional combat phase and post-combat phase.
The ruling on the Sphinx states clearly that
If multiple phases are added to the same point in your turn, the most recently created phase happens first.[...]
Thus, as I understand it, the added combat phase and post-combat main phase would take place before the additional beginning phase (adding a second additional beginning phase). Most importantly, since Aggravated Assault can only be activated at sorcery speed, I can't use the land which is untapped during my additional untap steps to chain combat phases, post-combat phases and beginnings phases into eternity.
Most other people seem to argue that the beginning phase would happen before the combat phase, or at least before the additional post-combat main phase, hence chaining phases forever should work. Which is correct?