Justin Bieber and Selena Gomez -- It's On Again ... Or So it Seems

Justin Bieber and Selena Gomez were at it again, and this time there were unintended beneficiaries ... some random people at a hotel bar who got to hear Justin serenade Selena for free.

The Biebs plopped his butt on a piano stool at the Montage hotel bar in Bev Hills Friday night and sang "My Girl" to his on-again-off-again-on-again-off-again-on-again-off-again GF.

After the mini-concert, they strolled through the empty streets of Beverly Hills.