5 Seconds of Summer just wrapped the North American leg of the Sounds Live Feels Live Tour, and the guys are having some fun with their time off. While Ashton Irwin and Calum Hood took some time to chill, Luke Hemmings and Michael Clifford headed out to the pop-punk summer camp known as the Vans Warped Tour.

Brian Dales, lead singer of The Summer Set, hinted that the band's Pomona, California, set on Sunday (August 7) would be "full of surprises," and as soon as fans spotted Luke hanging out on the side of stage, they knew something rad was about to happen.

"We're catching lightning in a bottle / Don't give a fuck about tomorrow," Luke sang with the band during the last chorus after joining them onstage.

"Some dude sang a song with us yesterday & now my socials are trippin, anyone know who that was???" The Summer Set's guitarist Josh Montgomery joked on Twitter.

Though Michael didn't perform, he was spotted by fans walking around the venue checking out some of his fave bands, such as Warped vets Yellowcard, 5SOS's Australian tour-mates State Champs, and up-and-coming pop-punk band With Confidence.