By now, I bet most of you have heard about the flash mobs pop­ping up at malls all over the coun­try dur­ing the Hol­i­day sea­son to sing the Halle­juah Cho­rus from Handel’s Mes­siah. (I’ve blogged about sim­i­lar events before here and here.) They’re fun and great PR, but also…dangerous?

Two months after a Cal­i­for­nia mall was closed fol­low­ing a dev­as­tat­ing fire, author­i­ties had to close it again Mon­day night after Christ­mas singers clogged the shop­ping center.

“The enthu­si­asm for the Hal­lelu­jah event tonight was far greater than antic­i­pated by orga­niz­ers,” said a state­ment from the West­field Gal­le­ria at Roseville. “Out of an abun­dance of cau­tion, the cen­ter was evac­u­ated. We apol­o­gize for this inconvenience.”

The Sacra­mento Choral Soci­ety was hold­ing what police called a “well-publicized” event to sing the “Hal­lelu­jah Cho­rus” in the mall’s food court, sched­uled for 7:30 p.m. (read the full CNN arti­cle here or an arts administrator’s take on it here)

The trou­ble here is that the orga­ni­za­tion pub­li­cized a “sur­prise” event. While yes, you do want peo­ple to show up, that’s really not going to be too big of a con­cern dur­ing the high point of Christ­mas shop­ping sea­son. This is what hap­pens when a bril­liant con­cept goes bad:

“At 6:45 p.m., the Roseville Police and Fire com­mu­ni­ca­tions cen­ter received reports that peo­ple in the food court heard pop­ping noises and felt floor move­ment,” a police state­ment said.

Hav­ing the police called in to shut down their event is prob­a­bly not what they were aim­ing for! Les­son learned here: if you have to pub­li­cize a “sur­prise,” don’t broad­cast it on the home page of your web­site. Tell select media out­lets or select groups who may join you at the event instead. For safety’s sake.