So, we know Alpert lingered around Locke's birth and later had the "test
encounter" with him. Yet this season we learned that Locke told him
about his birth. It wasn't some Dali lama type search Alpert was
conducting, but rather something he was directly informed about
himself. This sets up a whole pre-destination paradox for Locke.
You can reason this away by two thoughts. 1) Since that scene was a
flashback, it stands out from the others. Or, 2) The encounters are
known, but the characters do not acknowledge it. Why? Perhaps those
who experience the time travel learn the dangers of it and don't speak
up because they feel they may disasterously alter the events of time.
There may be some code they adopt. Perhaps a new form of "the lie,"
albeit a silent one.
For the second one to be true, Rosseau sticks
out. But I won't discount the possibility that she's time travelled
before. She's been on the island awhile and there is more to how
Danielle got to be Ben's daughter too.