I've read a few books lately where characters seem to teleport into and out of scenes on a regular basis. You think you left so-and-so back in the restaurant, and suddenly they're in the thick of the action ... but where'd they come from? Were they there all along, or did they just appear? Readers need to know.
So, when it comes to self-editing, take a close look at each scene and/or chapter, and at the characters who appear in it. Is it obvious that they were present all along, or are they lurking in the corners, doing their best wall-flower impressions, only to leap into the action when they're needed most? Continuity is important, as it prevents readers from being pulled out of the story by awkwardness, and a little bit of attention to detail in instances like these can go a long, long way.