Re: Fringe Question.... Potentil spoilers for seasons 5
The only thing I can think of is the existence of Henry. September explained that Peter having a child with the wrong Olivia creates a son instead of a daughter and prevents the future he came from (which is why the Observers wanted Peter to wink out of existence). They never actually explain what it is about Henry that prevents the existence of the Observers, and I imagine he was killed along with the rest of his reality in that alternate future.
Silent Night, Sleepy Night...