According to the book, he hit her for the first time when the boys were 8 months old.

Not to butt in, but in the book (which the show adheres to pretty greatly), Celeste says that the first time Perry hit her was when the boys were 8 months old, and she threatened to leave if he did it again. Then he hit her for the second time when they were around 2, but she didn't leave b/c the boys got really sick,…

I agree, I think she slips up a word or two because she is so emotionally involved in the scene. It doesn't bother me, her performance is brilliant.