On my first weekend job as an extremely untrained PFY, I was told to just do what I normally did for the same client during the week - look after the Mac kit on a bus/ring network during the day, make sure the OS X server was happy, then switch off and lock up when I leave. With a line of sight to the doors, I could live the life of Larry sitting in the server room doing whatever I liked, like fixing up an old Mac II to play Risk clones on or messing around with the likes of Photoshop and Captivate that I would never afford.
If someone quiet came in, and I was otherwise engaged, I wouldn't see or hear anything different if I missed them coming in the door. The server room was out of sight of the main room, and they hadn't told me anyone else was in - I couldn't see anyone else on Sunday night and wasn't expecting anyone. However, there was indeed someone in doing a sensitive sequential backup. As I left for the night, sending the shutdown command (which had been set to be ignored this time) and flicked the switches, I'll never forget the blood-curdling "NOOOOOOO!" my long haired silent colleague screamed as I wiped out his day's work.
Oddly enough I didn't lose my PFY status, at least until the long haired silent colleague became the long haired silent boss, but I never got asked to do weekends again.