(Yahoo!) - Leave it to Bieber. Not only did pop's most notorious/annoying bad boy make a surprise appearance Sunday night at South by Southwest, but he serenaded on-and-off-again girlfriend Selena Gomez from the stage.

Bieber's acoustic set at Banger's Sausage House & Beer Garden featured "Boyfriend" and "As Long as You Love Me," the latter of which he dedicated to Gomez. "This next song goes out to my baby," the Biebs said, before launching into the tune. And Gomez, who is back on with Bieber, was there to take it all in.

To say it was all a shock puts it lightly. Although SXSW features all kinds of acts, the festival is primarily known for showcasing the next indie-rock darlings, with the notable exception this year of Lady Gaga's planned performance and keynote speech set for Friday.

Journalist María Núñez summed up the surprise up best with this selfie:

Based on the video of the performance that has surfaced on YouTube, the show was pure pandemonium.

Prior to his SXSW appearance, Bieber and Gomez were spotted together at Starbucks in Texas on Saturday and he attended her show at the 2014 BorderFest in Hidalgo, Texas, on Saturday night before heading to Austin.

The Biebs's surprise SXSW performance capped a "Sunday Funday" showcase that featured other artists managed by Scooter Braun. Of course, Bieber is Braun's most famous client.

Photo Credit Getty Images