I don't have a DVR, so I can't check this, but I thought during The Subtitute that there are a number of times where there is a minor "flash" of some kind while following Locke in the Flash-sideways timeline. Sort of like a mini-version of the time flashes, with a hint of green. None of them result in a jump back to the Island time line, they are just like little wash-outs of the screen.
No one has mentioned this, so maybe I imagined it. Like I said, I can't go back and check. I think one happened while talking to Helen. One at the temp agency.
If I catch the replay I'll try and double-check it.
I thought maybe this is an early sign of whatever relationship between the two timelines there is. I certainly expect it to not be as simple as a split from the point of the bomb going off. I expect some sort of leakage, or decay, or something. The lines are in someway parallel and concurrent right now, I think, and that is going to mean something.