Don't underestimate the attraction of Twilight.
They only expected 300 to 500 fans to show up at a small Hot Topic store in a San Francisco mall where Twilight star Robert Pattinson would be doing a signing. So the lone security guard on hand was caught totally by surprise by the more than 3,000 who actually turned up. And then things really started to spin out of control, as disputes over who would get in culminated in a melee that left at least one girl bloody and others trampled. The San Francisco signing was cancelled, but the Hot Topic tour is scheduled to continue with cast appearances throughout the week, and a similar event in Seattle went off without a hitch.
TMZ has video of the crowd, and KCBS has an audio report with first hand accounts from the scene.