A few weeks after Shawn Mendes performed a beautiful cover of Ed Sheeran's "Castle on the Hill" [1], the British singer-songwriter returned the favor by popping up at one of the 19-year-old's concerts for a surprise duet. The pair, both armed with acoustic guitars and their ridiculously gorgeous voices, gifted fans with a duet of Mendes's "Mercy" during his Illuminate tour stop at Barclays Center in Brooklyn, and it left everyone in the sold-out arena with their jaws on the floor. Listen to the performance (and the sound of thousands of fans going absolutely nuts) above.