One of the most famous JLA issues ever!
The Story: "Metamorpho Says No!" by Gardner Fox, Mike Sekowsky, and Bernard Sachs. The JLA decides to induct new hero Metamorpho into the League("Yay, another Ramona Fradon hero!" Aquaman is overheard to say), but he and the League are attacked by the Unimaginable, a formless creature who demands to be a inducted as a member instead.
Roll Call: Superman, Batman, Flash, Green Lantern, Atom, and new member Meta...oops! Spoke too soon!
Notable Moments: This was a pretty surprising development--the JLA was the Knights of the Round Table of the DCU, so it seemed, er, unimaginable that someone would say no, especially when Hawkgirl nagged Hawkman every day, asking when she was going to get to join.
But say no Metamorpho did, and even though he looks good working with the JLA(there aren't too many orange and purple superheroes), he doesn't join. No other hero would turn down membership until a decade and a half later, when Black Lightning would have the same answer(hmm, Batman must have been making notes for possible members of a new team he wanted to start up...)