This happened with my onsite interview as well. I was working on local, user-based systems and hadn't done anything with networking/web-scale/sharding/etc.

I did very well with data structure questions, and had one difficult interview and one question about sharding a globally scaled system. I had never dealt with anything on that scale, and didn't even know how to think about it.

I realize that there was a gap in preparation, and I'm confident that that one interview out of 8 is what nixed my chances.