Not really a retcon, but my SCRPG campaigns have consistently had a big concert as part of their history where a super-musician accidentally gave a big chunk of the audience and crew powers, some of which wore off, some of which didn’t. So far that one incident has spawned eight different villains, three NPC sort-of-heroes, two PC heroes, and an indeterminate number of other folks who slipped through attempts by the government to identify and (if possible) de-power concert-goers.
There were no lightning monsters or even a giant space whale involved, though.