I had the exact same experience. 30 minutes paired leetcode type test of moderate difficulty. My solution seemed algorithmically sound and efficient, but the test wasn't passing at the buzzer. The guy failed me and we said our goodbyes. As soon as we're done, I slowly look at the problem without the distraction and see that I just need to swap two variables. I email the guy back barely a minute after we hang up, explaining that I just figured what was missing. He emails me that he's sorry I have to try again in a few months. Three years later I'm here on HN learning that I dodged a bullet.