this is my theory- back in season 3, des flashed into the sideways reality when he turned the failsafe. my logic for this is that he was blasted with EM and ended up in a strage world which he had lived before, but he could also change events without affecting his future in that timeline. also when widmore dosed him with EM in season 6, the FST is where he went, so that would be consistent with this theory
also, eloise seems to have crazy knowledge of the future in this episode, i believe this is because she had already experienced her "island revelation" and so was aware of a life she'd lived before and had to keep des on his path in the OT so he could fulfill his purpose
this could be very wrong, of course and would be happy to be proved wrong if anyone thinks of anyhting i missed?