...especially in Shanghai around the touristy Nanjing Rd. shopping area (also common in Beijing): The "tea shop"/"practice English" scam. There are variations but the basic scam is that you're befriended by a stranger who recommends going somewhere to sample some tea, practice English, etc. What you don't know is that the person is paid a commission by the shady tea shop or restaurant, which tricks you into paying a RIDICULOUS amount (often hundreds of US dollars).
Good luck and safe travels!
[ Edit: Sorry, no promos please. ]