I've just found something interesting:
In Superman (volume 1) #349 (in a story entitled "The Turnabout Trap!"), Superman returns from an interstellar mission to find that everyone on Earth are of opposite sex. Among them are Penny White (a female Perry White), Jenny Olsen (a female Jimmy Olsen), Louis Lane (a male Lois Lane), Batwoman (a female Batman, rather than the actual character), Wonder Warrior (a male Wonder Woman), Black Condor (a male Black Canary), and Superwoman (his female counterpart) herself. Believing he crossed into a parallel universe, Superman flies back to space to find a dimensional portal, but is blocked by an invisible barrier. He notices the parallelism fails when he sees Superwoman and Clara Kent (Superwoman's presumed secret identity) are two separate people.
After a battle with Superwoman, Superboy (a male Supergirl), and Wonder Warrior, Superman figures out that his foe Mr. Mxyzptlk is behind this gender-reversed world; Superman discovers as well as that he was never in a parallel universe, but rather on Earth, which Mxyzptlk had altered with his magic. After making Mxyzptlk say his name backwards and thus returning to his native dimension, the effects of Mxyzptlk's magic (including the existence of Superwoman) vanish, returning the Earth to normal.
(from
wikipedia )
