Once I logged in my wallet at one of the payment systems and saw the message that sounded somehow like this "User@@@ have transferred 500 dollars to your account by mistake. Please, return this money to him. Your account will be frozen until you return the money." I noticed that the sum of money I had on my account was really 500$ higher than necessary, that is why I decided to do what they asked me for – to resend 500$ to indicated user. Thanks God, this payment system has implemented two factor authentication service by Protectimus recently, and I had to use an Android app Protectimus Smart in order to receive a onetime password, when I entered this password, immediately appeared a warning about hacker's attack and 500$ remained on their place. As I have read later, there is a kind of modern hacker's techniques, when the information the user sees on his screen doesn't coincide with the real information, this is the method I could get caught.