Fall Out Boy performed an impromptu gig to showcase their new album - but ineffective paperwork meant the band were forbidden from playing their instruments.
The rockers were in Washington Square Park in New York on Monday (15Dec08) to film a jam session in front of a small crowd.
However, a blunder with the administration meant they had to play without using their equipment.
But the group soldiered on - and played the set acapella, according to PerezHilton.com.