It's rare that you will find anyone that says positive thinking is a bad thing. However, we see positive thinking being used as a tool to manipulate people to buy into scams. All the work-at-home money-making schemes have a common theme. They use the very words that our parents used to tell us, "Believe in yourself, and anything is possible." Schemers make it even more alluring by giving the impression, "Not only is it possible, it's likely that you will become rich using this program!" Instead of using this idea to spread goodwill and confidence, scam literature uses "positive thinking" to trick people into believing that they will "get rich quick." We always urge people to check the validity of these programs (99.99999% of which are scams), but they often check into it after having already purchased it.
I ran across an interesting video about how people can be manipulated by messages that promote positive thinking.