Revealing mistakes: The camcorder would not be able to play back video in slow motion, as we see in the early frisbee scene. The shot was clearly made on film, overcranked, and then transferred to video to display on the LCD screen.
Revealing mistakes: When Johnny is carrying the air conditioner, he turns and we see through the grill that's it's empty.
Revealing mistakes: In the ball throwing scene at the carnival, as Johnny throws the last ball and the camera goes to a blurry slow-motion movement, you can see that the ball is still in his hand as it goes down yet a ball is seen entering the hole.
Factual errors: The air-conditioner is 220V. Even with a new plug it would not have worked in a 110V socket for any length of time.
Errors in geography: In the shop in New York, a neon sign in the background reads "Take Away" which is actually the European phrase for take out food, and the sugar packets on the tables are tubes, which are also used mostly in Europe.
Continuity: After seeing Mateo at the hospital, Johnny's position on the stairs changes between shots.
Continuity: When talking about Christy's blood, Johnny's position changes between shots several times.