I've noticed recently that, after getting Ambitions (I'm broke, so I'm way behind with expansions, lol), I notice other townies that seem to be under the pavement.
I've mostly played Sunset Valley, and more often than not when one of my Sims is in town, I'll see speech or thought bubbles on the ground; even worse, when these Sims talk to my own, I have no way of responding to them.
I suppose it's a silly question, but my Sims' romantic interest fell victim to this "sinking", and I had no idea how to fix it. I was tempted to lure them into an open area and use a Miner (that was my attempt at a joke, by the way).