I read on lonely planet that around duome tourists must be carefull, and it alsmost happened to me.
I took metro red line from Duomo, and when I was about to enter the metro, suddenly 1 big guy (185ish looks like eastern eu and seems to always wear something dark) bumped me from behind and get in front of me. When he was inside the metro and I was about to get in, he pretended to drop his sunglass, and he grabbed my ankle, I dunno for what, and he started to say something in italian to me.
Then I just remembered that the day before at the same place this guy with I think at least 1 of his friend trying to enter the metro when the door was almost closed, and they were inside for few seconds and get out again just when the door closed.
So when he said something to me in italian, I just said no no no, and I tried to get out from him and get out from the metro and move to another compartment.
I dunno his trick or his scam, but Im sure that he is dangerous for tourist.