Continuity mistake: After the explosion in the laboratory, Josh is laying on the floor, injured. His face is covered in blood, however during the scene the amount of blood on his face keeps changing. In close ups of his face his lips are free of blood, but in wider shots his lips are covered in thick amounts of blood.
Continuity mistake: After visiting the home of the Djinn, Alex receives a call from him and answers the phone and screams into the receiver without pressing the "talk" button on her cordless phone, so her response wouldn't be heard.
Continuity mistake: Towards the end of the Beaumont party scene, Alex and the security guys are in the hallway with the killer statues. One of the security men falls to the floor and his earpiece falls off. Throughout the scene, it keeps changing position from being on the floor next to him, to back on his ear, to being on his ear in a different position.
Continuity mistake: When wishmaster is causing chaos at the party, we see an actress in a green dress get killed and fall on the floor. Later on, we see the same woman running out of the house with the rest of the crowd.
Revealing mistake: After the Djinn escapes and there's an explosion in the lab, you can see some stuff on the ground that looks like it's on fire. The objects are not actually burning, the fire is being produced by a burner. There's no smoke from the objects, which is a dead giveaway that they're not actually "on fire." (00:18:15)
Answer: There's no known answer to this, but one thought is it would be very difficult for anyone, at some point and without thinking, not to wish for something, particularly something frivolous. Something like, "I wish it would stop raining." Theoretically, the Djinn could then use a "loophole" by granting the person's wish three times without them realizing it. If the person somehow never wished for anything, then presumably the Djinn would remain captive.
raywest ★