In The Abominable Bride, Sherlock spends most of the episode in his mind palace and re-creates the case of Emelia Ricoletti, (almost) exactly how it was. But there are dozens of independent people in his mind. Is he really creating consciousnesses in his brain, or how does that work?
Secondly, near the end, Moriarty says
Too deep, Sherlock, way too deep. Congratulations, you will be the first man in history to be buried in his own Mind Palace.
Could that actually have happened? Would he have died/gone into a permanent coma?