The cause of my worst incident, which happened in a nonMarriott property, was a room change/upgrade at check in, with the front desk agent apparently failing to correct my preassigned room in their system. An hour later, someone tried to rattle the door when their keycard didn't defeat the deadbolt; I was taking a bath at the time.

A scarier but related experience was when the check in agent gave me keys to a room that had a connecting door that was closed but unlocked. Fortunately I discovered this during my initial reconnasiance to check that windows in my room were properly locked. However, the front desk could not seem to understand why I was upset and wanted that connecting door locked immediately--which required maintenance to come with the right key to the dead bolt as this was an older European property--when they told me that the people on the other side seemed nice.