What do you do when your girlfriend calls you by her ex-husband's name?

Confront her about it. She may not realize she's doing it, and it maybe that she's not doing it on purpose. Talk to her and got to the bottom of why she's doing it, and see what you two can do about it. You have to understand she was once married to that person, it's not the easiest to just forget someone you once developed strong feelings for. And depending on how long your girlfriend was married to her ex-husband, it could have just been a habit she developed over the time period she was with him, and it could be a possibility that she may still have that habit in her. As long as your communicate with her and tell her you're confused by her doing that, or you're uncomfortable, i'm sure talking about it will help the both of you out, and hopefully she'll stop.