why did you say that?

Why, oh why?

Story is frolicking along, heroine is waking up in a strange location and talking to her hostess, and the hostess goes and says that it's not the first time that strangers have shown up like that. . .

sigh.

Which means that I've got to figure out what the other time was, and who it was, and how it ties into the events that are happening now, to make it significant.  Only in real life can people talk like that and not have it be significant.

My outlines have a tendency to only track the main plotline.  So subplots have a marvelous tendency to spring up in the middle when I realize that a throw-away point doesn't work if it's just throw-away; it needs to have importance.  Or I stick a bit of characterization on a character, to lend him some color, and he goes, You know I did this, that, and the other thing, too?  Not to mention that I obviously got introduced three chapters ago. . . .

At least this plot development only affects the story going forward.  Going back to introduce all the other elements of a subplot after you've written the climax can be fun.