Shortest Substring Index
Pyth, 39 bytes
Lsm.:bdtUbKfT.zj\,mhf!}Yjb-Kk+yky++bkbK

Bruteforces all subsets of every string in increasing length and checks if that string occurs inside any other. If that doesn't work, it will do the same except for all subsets of \nstring\n.