So my cofounder was talking to Stripe about an acquihire (this was after I’d left.) As part of it, he had to do a systems design interview.
He got the prompt, asked questions about throughput requirements (etc.), and said, “okay, I’d put it all in Postgres.” He was correct! Postgres could more than handle the load.
He gets a call from Patrick Collison saying that he failed the interview and asking what happened. He explained himself, to which Patrick said, okay, well yes you might be right but you also understand what the point of the interview is.
They made him do it again and he passed.
Sorry, I didn't get it. What was the 'right' answer?
Sometimes you just have a bad interviewer who is looking for something specific from you but isn't telling you. If you're experienced in these interviews, you catch the signs and adapt by asking questions to suss out which direction the interviewer wants to take it.
Sometimes your answer is plausible but the interviewer wants to see you justify it. Sometimes your answer is wrong but the interviewer wants to see if you can reason your way through it, and maybe come up with an alternative.
If you're junior/inexperienced, it's often hard to tell and it'll feel arbitrary/unfair, and unfortunately that's just how it goes. As a more senior/experienced candidate, you can often figure out which situation you're in by asking questions to feel out the interviewer and then try to pivot during the interview, though it still takes valuable minutes out of the interview that you could have otherwise spent showing your competence.