An update addressing an issue where players may not be able to complete the mission "The Once and Future Slab" 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.