HIMYM fic: Caught!

.... Finally, Barney broke first, ending the stand-off. "Damn it, Lily! Could you at least call first before bursting in unannounced?" Still recovering from the shock of seeing Barney doing something so completely unsleazy, and a bit relieved at that, Lily had to shake her head to respond. "Sorry, I-" Lily awkwardly fumbled, then latched onto the first thing she could think of. "Hey, what song ...