Had an interesting thought I'd like to share, and apologise if something similar has been posted already.
We know there are two timelines going on. How this happened and why is still a mystery. Here's a theory..
So let's say that there are not just two, but an infinite number of "timelines" existing. When a person makes a decision, the timelines split. For instance, I decide to have breakfast in the morning, which is Timeline X. Or, I decide NOT to have breakfast in the morning, which splits off from Timeline X into a new Timeline Y. And before anyone gets upset, just hear me out.
Perhaps two of these infinite timelines managed to criss cross at the point of turbulence on Flight 815, literally due to massive electromagnetic interference (caused by Desmond not pushing the button, this time). At the exact point of.. let's call it "interception" between the timelines, parts of both timelines began to bleed into each other, in a way. For example, Jack and Kate recognise each other briefly outside LAX, maybe even having their own flash-sideways to the original timeline that we don't see, much like how Desmond has his flash-sideways to the original timeline when he sees Charlie in the car underwater. This could also be why little things like Mikhail being shot in the eye occur.
This is a pretty bare theory at the moment, so I'd love to hear everyones thoughts and comments.