An update addressing an issue where the mission "Stalker of Stalkers" is not considered complete upon Turn In has been released on PC, PlayStation 3, and Xbox 360.
PlayStation3 and Xbox 360 users can download the update by playing Borderlands 2 while connected to PSN or Xbox LIVE, respectively. PC users can download the update automatically through Steam.
If you're still experiencing this issue after applying the latest update, please open a ticket for additional troubleshooting.