IIRC, their first reply was what they would type into a calculator, not “I don’t know”. Only when confronted of what the result would be did they say “I don’t know”, because any human with a life wouldn’t know off the top of their head.

Another example is from my own "worst interview ever" where the guy asked me to implement a BFS and a DFS on a tree. I mentioned that I'd literally been debugging an issue with a DFS algorithm all day at $dayjob, so should be a piece of cake. I wrote out the DFS example in 10s, but then when I got to the BFS, I spaced out on how to write a for-loop. The interview literally just laughed at me, saying, "that sucks for you" so instead I simply showed him how a BFS works and his answer was, "I don't care, I want to see code."

As an interviewer, I get it. I expect you to know concepts, not be able to perform rote actions and put on a show.