Justice League of America #16 (2007) First Print, Cover A

"A Brief Tangent" Two small-time crooks named Alan and Paul break into a storage unit and find a green paper lantern. They unwittingly open a dimensional portal to the parallel world of Earth-9. The Atom of that world steps out, displacing Alan into the other reality. The Justice League are called in to investigate when it is revealed that the storage unit is leased to Guy Gardner. Black Canary, John Stewart and Red Arrow answer the call. A fight breaks out and Red Arrow manages to subdue the Atom. John Stewart realizes that the lantern that brought the Atom to this world is something that Kyle Rayner came across a short time ago. He had asked Guy to hold it for him. John uses his ring in tandem with the paper lantern to open a dimensional portal and he returns the Atom to his native world, while simultaneously retrieving the crook Alan. What none of the League yet realize is that the Flash of Earth-9 has likewise left her home dimension and is now on Earth.