A bodhisattva of a high degree of acumen does not regress after the first time he or she develops the supreme mind of awakening; a bodhisattva of average acumen does not regress after attainment of awakening’s first stage; one of lesser acumen, at the eighth stage. These are the three points [after which bodhisattvas] no longer regress. One teaching even states that those of very low acumen [no longer regress after the attainment of ] awakening’s ninth stage. These categories are based upon distinctions in capability.
WOW! Shakyamuni belongs to the first category. This is expounded upon in Dharmamitra's "Commentary on the Perfection of Sublime Insight. (does this exist in translation?)." This is quite different from the standard teaching that states that a bodhisattva does not regress after attaining the Path of Seeing (1st bhumi). To be able to regress even if one is one the higher bhumi's is stunning.