Most candidates who fail a system design interview know enough. They know what a cache is. They can explain sharding. They have watched the YouTube design five times. They still fail. Not because of what they know, but because of how they use it in 45 minutes. Interviewers score a short list of signals. Can you scope an ambiguous problem? Can you make a choice and defend it? Can you find the weak spot in your own design? Can you run the conversation? The seven mistakes below break th...