Continuity mistake: After Jack places the pig's blood on Roger's cheek, he then places it on his own. Look at the amount of blood on Jack's index finger. It goes from a little to a lot.
Revealing mistake: When the boys use the glasses to start the fire, there is smoke coming from under the wood they are lighting. This wouldn't happen. It would come only from the focused point of light made by the glasses.
Revealing mistake: When the three boys are sitting by the fire the footage is repeated. At first the fire is normal, but suddenly the flames starts going downwards. (00:34:50)