Jimmy Olsen turns on his best friend in Action 253 (June 1959), in a story by Alvin Schwartz, Curt Swan and George Klein.
Superman takes Jimmy on his first visit to the Fortress of Solitude, where he shows off Kandor, which seems to be in a much larger bottle than it was previously. Did he replant them?
Coming back to Metropolis, Jimmy is arrested by the police for not revealing a source, a typical situation for reporters in stories. But Jimmy will have none of it, and suddenly displays super-powers, using them to escape.
He winds up in a confrontation with Superman, who spends far too long being puzzled over his friend’s behaviour. Superman suspects the visit to the Fortress is connected with Jimmy’s change, and he is right. This is not Jimmy, but an identical, Kandorian, villain. He escaped using a body-transfer ray. Superman sets things right before El Gar-Kur can destroy Kandor. This Kandorian Jimmy never appears again.
Supergirl gets a gentle adventure in her first solo outing, by Otto Binder and Jim Mooney.
Because Superman has ordered her to stay at the orphanage, she scuttles her own chances of being adopted by a couple. But she goes out of her way to get one of the boys at the home some new parents.
When a couple who want to adopt the boy are rejected for being too poor, she drills a hole through the Earth on their property, which becomes a tourist attraction, and enables them to afford to take the boy.