A spontaneous musical concert at a California shopping mall ended with the entire complex being evacuated after some 5,000 people turned up to sing.
The incident happened at the Roseville Galleria shopping mall, which is reported to have started shaking due to the volume of people inside.
Fire and safety officials struggled not to spoil the festive mood as they evacuated the mall.
One warden said: "You can keep singing, but please walk."
Thousands had gathered at the shopping mall's food court to take part in a group rendition of Handel's Hallelujah Chorus as part of a flash-mob concert - when groups of people - often strangers - conspire on social networking websites to turn up at the same place at the same time and start singing.
But, as they burst into song, there were reports that the building's floor began to move and creak, and witnesses reported hearing popping sounds.
Emergency services were forced to evacuate the entire building.
"We want you guys to have a good Christmas but we have a serious safety issue right now," one official said, according to a video of the event posted on the website of the Sacramento Bee.
No-one was hurt and the structure was declared safe.
The assembled singers, whose efforts had been organised by the Sacramento Choral Society and Orchestra, continued their impromptu concert outside.