Never lend your spare key to a flatmate who "just needs it for the weekend" — mine copied it, let her boyfriend move in rent-free, and when I finally changed the locks she had the audacity to call me "controlling" in a group chat that included my mother, who screenshotted it and sent it to me with a single thumbs-up emoji. My father, predictably, suggested I should've drawn up a formal licence agreement before handing over a cut Yale key, as if a contract would've stopped someone from being fund