Goofs

After having escaped Sinestro's dungeon, Green Lantern (John Stewart), can be seen wearing the Green Lantern suit underneath his jacket, in spite of not having the ring. His suit originally appeared when he put on the ring. A minute or so later when he had re-acquired the suit is gone and when he puts the ring back on the suit re-appears. There's nothing that would indicate either a real suit had been put on, nor the time to take it off. It also would make little sense to do so.

John's hand scar switches hands multiple times throughout the film.

When John first gets the ring, it floats down to the palm of his hand then seemingly slips through his finger to end up on his finger. It's concealed by the ring's glow, but it does not slip over the tip of his finger as it should. Green Lantern rings are always shown slipping on and off like normal rings.

Sinestro twice conveniently forgets a Green Lantern's ability, which he would surely have learned as one, to summon his/her/its ring.