- I did the system design at the wrong level of abstraction (too low)

- Got asked about lowish-level database details. I took the question at face value and said I didn't know. What I should have done was explain what I did know (which was at a level slightly higher than the question), make an educated guess, and say how I would find the information. I still don't understand why Id didn't do that as I've always done it in interviews before or since.

This is my own assessment, they didn't give detailed feedback. Just that I wasn't strong enough in some areas (paraphrasing).