There have been a couple of times that I have deviated from my outline. I think I know everything I want to cover in a chapter but sometimes I will go off on a tangent and won't get everything done in one chapter before it gets to be unwieldy. That happened last night as I was finishing Chapter 12. Wow, my main characters covered a lot of ground and progressed a great deal in their relationship, but it meant I didn't accomplish some of the other stuff I needed to do in Chapter12. So now I'm looking at 19 chapters instead of 18 total.
And I'm not sure I can write a book with 19 chapters. I feel like I will create a chapter 20 at some point. I'm just weird like that.
I'm happy with the way 12 turned out...still left some questions unanswered...and ready for Chapter 13!