I'm sure I've told this story on here before but it's worth retelling...
2005, I was in Rome. I'd bought advance tickets to a movie being shown in English that night to make sure I could get in. Movie was at 10pm and I had a few hours to kill so I decided to spend them at the nearby Trinity College Pub relaxing since I'd been out exploring all day. After several pints I was pretty tipsy and decided I'd better head out to the theater which was at the end of Via del Corso, near the Piazza del Popolo. If you're familiar with Rome, you know that Via del Corso is the popular street for people to walk at night so they close it to traffic and it gets pretty lively. I was carrying my camera, my phone and a small paper bag full of Popeners (bottle openers with the Pope's likeness on them, I buy them as gifts for friends and family), which I had stuffed in my front pocket. Halfway down Via del Corso I get stopped by two uniformed Caribinieri who asked me for my passport. I explained to them that it was in my hotel room but they did not like that so they made me follow them to a nearby checkpoint where there was a big Caribinieri van parked. I don't know if I somehow looked dangerous or what but 5 or 6 uniformed Carbinieri surrounded me and I'm freaking out. The base I work at is technically a Carbinieri base so I showed them my military ID and told them where I work but they just stood there. Suddenly a well dressed older man in a suit emerges from the van and comes over to me. I try explaining things to him too, he says "It's ok, it will just be a moment...tell me, what do you have in your pocket there?" I pull out the bag, it rips and Popeners go flying everywhere on the ground while the Carbinieri guys quickly take a step back, startled. The guy in the suit looks down at the ground, says "Ah! Il Papa! You can go sir, sorry to bother you and buona sera."
So if you're not going to carry your passport in Rome, you might want to carry a few Popeners just to be safe...