Cobb County, Georgia. Gwinnett flipping was predicted widely. Cobb was mostly thought to be a cycle or two away.
Cobb was a shock, Gwinnett was seen as more realistic but Cobb was always thought of as this wealthy preppy white suburb that would flip unless minorities were 40%+ of the population OR a very 1990s bill clinton centrist type candidate came along.
I'm not saying this is the reason Cobb flipped, but many voters did see Hillary as a centrist. Whether that was by association to Bill, her [conservative] tenure as SOS, or undeserved is a topic for another discussion.