The Manhunters arrive on Earth via rockets in their boots, just three of them, but are beamed away. What is the source of the beam -- there was no ship or any more of them at Earth.
The Manhunters arrive on Earth via rockets in their boots and land that way, too, yet when they leave, they are beamed up.
The angry aliens on the planet are yelling and pumping their fists in the air at John, but when he flies over, they don't turn around and continue to yell and pump their fists at empty air.
In the scene where the big street battle takes place between the Manhunters and the League, the word "restaurant" on a local business is misspelled "restuarant".
J'onn J'onzz points to a display of stars in the galaxy, indicating what planet the Green Lantern was taken to. The holographic map displays the name as Juras 5. How would Batman's computer know the name aliens gave their planet?
When John takes his ring off, there would be nothing powering him any longer, so not only should his Green Lantern outfit disappear, but his eyes shouldn't be green either.