A couple of debates back, Warren and Sanders got into a spat over whether or not he had said to her America wasn't ready to elect a woman president. There apparently were no witnesses to the supposed interaction.
I don't know if he said it as Warren described, or if what he really said was America wasn't ready to elect Warren president and she assumed he meant it was because she wasn't a guy. Regardless of what he said, the prediction as she interpreted it was 100% correct.
Sanders was right; barring a black swan cataclysm, America will not elect a woman president in 2020. It seems odd no one has noted this prediction being proven correct so quickly. If Americans paid attention to what happens overseas - most don't - I'd blame Angela Merkel.