But there might have been more than just the pressure of success that was eating at Nguyen. According to a report published on Monday, the developer had received a letter from Nintendo. The Japanese game developer wrote Nguyen to warn him that the pipes used in Flappy Bird, along with some other background art, appeared to be taken directly from Nintendo's Super Mario games. The Flappy Bird developer had previously denied copying others in the creation of his game.
While Nintendo won't confirm or deny that it had sent a letter, Nguyen apparently avoided legal problems with Nintendo by taking the game down. But that might not save him from another problem. Another report out of Vietnam reveals that the country’s Ministry of Finance is looking into Nguyen's earnings to see if he owes Vietnam some income tax.
source: VNExpress (translated), Forbes