Oh wow. My mind drifts all the time. It's hard to pinpoint it towards a specific topic or area. I suppose it depends on what emotional state I'm in alot of the times. If I'm overloaded with work, it will be flashes of what has past or what might come. If I'm thinking about a specific idea, my mind floates all around that. What, where, how... I also get those "omg, what did I do/say. How could I have recovered better" flashbacks at times. Those are the worst, lol. Then, like you said, we move on.
It's awesome that we all relate to it.
I think of those awkward moments and what we would do differently if presented with the chance.. And If like to believe it our minds way of gaining wisdom.
With certain topics, it's like mental preparation for what it ahead or how to deal with what is past.