I think the parallels between Desmond/Daniel and Salughterhouse 5 (in the post of the same name) are right on. Desmond has been experiencing the phenomenon for some time, we've only seen bits and pieces of it through his flashes of the future and the key turning incident. For all we know Des could be in a constant state of jumping between points in his life, none of which he can change, just like the guy in SH5. Maybe the "side effects" only affected his memory of what takes place during the different flashes. He already (previous to this episode) went back in time and has had multiple visions of the future which came true. The only difference between Des now and Des then is that he can no longer remember/differentiate between different points in his life.