Although I am not at all a connoisseur in this area i know that counterfeits are relatively frequent - especially when we're talking about $15.8 million.
What is especially noticeable to me is the back which assigns that sheet to Leonardo da Vinci, being most likely original but would hardly sell for a price that high.
The front on the other side is - almost too perfect - too unbelievable, to good to be true.