Once I just got off the metro, and was on an escalator in Barcelona. I felt someone was touching my backpack so I kind of moved it around a bit. There was a wild looking woman behind me (but not apparently a thief). Later I found out that my backpack was unzipped for the most part. But nothing was stolen. Because I alerted her that I was paying attention.
When I was in Paris once I got on a train, and several people to my left were making some strange moves as if it was too crowded than it actually was. Suddenly a man on my right was also blocking me from moving, and I found that my bag was half unzipped. I felt the man was digging in my right pocket of my pants. I immediately slammed my pocket and a plastic bag in his hand went flying. I did nothing and he got off.