A bug with the Bard feat Subsonics.
When activated, all song attempts cannot be sourced, but allies/enemies receive benefits/drawbacks.
From invisibility, it seems that casing bard song attracts the attention of hostile enemies as if you dropped something or cast a spell. Simply go invisible, turn subsonics on, and stand near a pack of enemies. Then just use bard song:
This also occurs from stealth. And using bard song while subsonics is active from stealth, throws you out of stealth when nearby enemies, and no rolls are made. As I understand, this is supposed to work while in stealth.