Blizzard freaks out about changing IP addresses when you sign in to World of Warcraft. I have two internet connections at home and sometimes I switch back and forth, and back when I was playing WoW, it would lock me out after each switch. I had to login to my account via the battle.net website and stell it that everything was ok.

It was extremely annoying. The last few times it happened I just didn't play for a while because I didn't want to bother with reauthenticating; that's when I realized that I didn't enjoy the game that much any more and cancelled my account.

for the sake of counter-example: facebook does. E.g. if you try to log in from a different country than ones it already knows you have to go through a long process of reauthentication by inputting your friends' names based on pictures. At least, it happened to me twice.