Bugsy Siegel and Steelgrave:
The Little Sister
The plot of The Little Sister turns on a photograph cladestinely taken by Orrin Quest of an ex-mobster named Steelgrave who was supposedly in jail at the time a rival gangster was gunned down.  Quest's photo shows Steelgrave dining at a local restaurant the day the rival was murdered, effectively ruining Steelgrave's alibi.  Quest is using the photo to blackmail Steelgrave, and this blackmail plot ultimately gets him killed.
Chandler likely got his idea for this plot twist from a real life incident involving Benjamin "Bugsy" Siegel.  Siegel--like Moyer/Steelgrave--was an Eastern mobster who came to California, tried to clean up his image, and made friends with Hollywood actors and producers.  In August 1940 Siegel was arrested in Los Angeles for the murder of Harry Greenberg, a small-time gangster who had threatened to inform on his bosses.  Siegel received special treatment in the County Jail, being allowed free run of the jail floor, unlimited use of a telephone, and the delivery of meals from outside restaurants.  He was also given passes to leave the jail some twenty times, supposedly to visit his dentist or meet with his attorney. 

Mobster Benjamin "Bugsy" Siegel

 

 In the fall, reporters for the Los Angeles Examiner got wind of Siegel's absences and began tailing him when he left the jail.  One morning, dressed in an expensive suit and silk tie and accompanied by a deputy sheriff, Siegel visited his dentist then stopped off at Lindy's Restaurant on Wilshire Boulevard, where he met British actress Wendy Barrie for lunch.  The Examiner photographer snapped shots of Siegel and the deputy leaving Lindy's, and the story was run on the front page of the next day's paper.  The expose sparked a grand jury investigation that culminated in the firing of Dr. Benjamin Blank, the County Jail doctor who wrote Siegel's passes.  It was later revealed that Blank had received some $30,000 in checks from Siegel, though the doctor claimed Siegel was merely repaying a loan and not bribing him for special treatment.