If this was a junior candidate or something, I may have let it slide. But this was a senior candidate, which means they are supposed to be a technical leader for the team. I can't have someone in that role with such fundamental misunderstandings. There were more red flags than just that one, I also don't fail people for one gaff (e.g. I just passed a senior that bombed the coding challenge, but it was obvious they were over-thinking it).