There's nothing like flash bitcoin, they may engage in making a fake transaction by showing a newbie that they have sent it and it went through and even showing them previous transaction id, but a newbie may not understand how to verify that through the use of block explorer or on the mempool, some also use the trick in double spending to deceive newbies as well, all i can see is that this kind of attack can only be done to beginners if at all it exists, because bitcoin transactions are irreversible, but in this case, they wouldn't send it, but yet claim they have sent it to newbies in other to scam them.