I got trapped behind an invisible wall
In the Questionable Area, I rode a log to near the top of the waterfall and jumped off. This resulted in me being on the right side of the waterfall (facing it) but behind an invisible wall that's supposed to normally prevent me from going up there. This was effectively a soft lock as there was no way out of there other than to close the game and reload from my previous autosave (also why can't I manually save?).