Things I have no evidence to back up:
Blue house syndrome - seeing one person paint their house makes you paint yours then others do it, then others, then others, so on.
Jack the Ripper was really multiple people seeing the first murder and figuring "hey, I can do that", creating multiple copycat attempts and making a sport out of killing prostitutes.
The Total Eclipse - an event hidden by the occurrence of a larger incident.
The presses focus on the Jack the Ripper slayings resulted in another brutal killer going unnoticed. Whitehall. Pinchin.
Life imitates art - a fan takes things one step too far.
The incredible performance of Richard Mansfield in the Strange case of Dr Jekyll and Mr Hyde inspired a failed surgeon to re-enact the events of the play on the streets of London.