Gearbox says the most recent update to Borderlands: The Pre-Sequel, which launched today alongside the Lady Hammerlock DLC pack, has an "unforeseen issue" that's causing the game to crash for some players.
The problem occurs when players put three items of purple rarity into the Grinder, which "has the potential to cause a crash," Gearbox wrote in a Knowledge Base notification. The studio is working on a fix, and in the meantime it recommends that you use the Grinder at your own risk, although I think the wiser recommendation would be to just avoid using it at all until the patch is out the door.
The update, unfortunately, is automatic, and required for online play and access to the Lady Hammerlock DLC. You can avoid it, however, if you're prepared to do without those things by putting Steam into offline mode before you play—not the most elegant solution ever, but hopeful also a very temporary one. Gearbox said it will post information about a fix on the Knowledge Base as soon as it confirms that the problem has been solved.
