Always check your math. And then go back and look at your math.
I am working on a book for Chelsea Publishers. I did the math and chapters 2-10 need to have approximately 7200 words each in them. For some reason, my head decided that should be 6200 words.
I have been cutting and cutting trying to get the chapters that short.
Oops.
Now I need to go back in and bump up the work.
The good thing is that it will probably make the chapters stronger, but that was stupid.