To be fair, it's not really unprofessional. But two weeks ago we recorded a theme song for our company in an empty office room. Though I shouldn't complain, I love doing those kinda things :)

A couple of support engineers ditching daily standups in the name of meeting and doing lunch/snacks (came to know when me and other teammate saw them at the cafeteria). Maybe not that unprofessional, but annoys me anyway. Never lie.