It seems that Christa headbutt ver is very popular... Already forgot which chapter Christa did that... hmm...
I think it was chapter 36 . the one where Sasha goes back to her village. while saving a kid she has a flash back of Ymir judging her for speaking formally instead of using her normal dialect. Christa feels that Ymir says a bit to much and she head butts her two times on the same page. This chapter was also the chapter Ymir is named, so it may actually be where it started.
