You’re writing a scene with an event that should strike your main character as surprising or distressing. BUT instead your character reacts in a way that makes no sense at all. Maybe you had your character have to shoot an intruder and it was the first time they had to shoot anyone. How would a person normally react in a situation like that, if it was their first time having to shoot someone?
Make sure your character who is going through that for the first time reacts the right way. If, for example, they walked away from the above situation behaving as though it was no big deal, then there better be a logical reason for them doing so. I say this because I know I would freak out if I had to shoot someone. Or I’d panic. Maybe others would become despondent or go into shock.
If your characters reactions don’t match the situation, the readers will know, and they’ll get distracted from the story. You don’t want that. It could even cause them to put it down, and you definitely don’t want that.