Katsuki and Izuku are so used to calling each other by "Deku" and "Kacchan" they've never called each other by their first names like never! So I can just imagine them going through a list of names looking for the others name and not finding it but it's right there under their noses! Katsuki would be so mad, because he gone through whole list and didn't find Deku's name until realization hits him. Same for Deku. Erm its just something I feel like they've done or do XD

HAH I can see that happening. They’re like, “Oh yeah, he has a first name… An actual name… Right.” BUT when they finally do use each other’s first name, they both get extremely flustered and it’s adorable.