16mm one day wonders, typically known for shoddy production values, minimal plots, and running times of an hour or less, were among the first and most lucrative types of hardcore movies to be produced, starting in 1970.

While all three major production hubs for hardcore, LA, NYC, and SF cranked out thousands of mostly throwaway pieces of single location schlock, the films coming out of SF were, at least on a relative scale, generally more interesting than their SoCal or East X Coast counterparts. The following is a survey of three such films produced in the early/mid 70s in San Francisco:
The Unholy Child (Jim Turner, c. 1972-3)
Playing off of some deliberate giallo tropes, The Unholy Child is a murder mystery without the murder, but that's not for lack of trying. The film's Fu Manchu looking would be assailant keeps attempting to kill the protagonist, but fails every time. Even with a bomb! 
Nevertheless, The Unholy Child plays a bit like a love letter to The Tenderloin in all its glory as its cast of drug addict looking one timers very earnestly trudge their way through pleasantly filthy locations while trying to make sense out of a plot that's kinda senseless.
Director Jim Turner probably never made another film, but his attempt to produce a gritty urban thriller works beautifully because all of it feels reel. The locations, the casting, and even the kinda lame ending all work in creating a dramatized window into the lives on early 70s white trash. 
Left at the Altar (Connie Sullivan, c. 1973-4)
Technically more ambitious than what Mr. Turner was capable of, this downbeat drama follows a young woman's misery filled love life, until the film reaches its foregone and title implied conclusion.
Like Child, Altar is a film that works on account of the authenticity of its casting and locations. Mystery director Connie Sullivan pleasantly mixes weepy style melodrama with touches of inner city living, only to be somewhat undone by a lengthy sex scene that runs around 8 min longer than it ought to.
But still, accepting that the film had to concede a bit to audience expectations, what we get isn't half bad.
The Hit (Bob Kirk, c. 1976)
If I were hard pressed to pick the single least talented director in sex film history, I'd probably choose perpetual hack, Bob Kirk. His films convey a laziness that would likely have infuriated even Carlos Tobalina, but like Carlos, Bob loved self promotion, often narrating his trailers and repeating his assumed name non stop, and even going as far as to provide an infomercial style coda to at least one film, in which he reminded audience members to come back soon for his next product...er...movie.
Anyway, The Hit is typical Kirk fodder that, after a brief exterior sequence, becomes a 55 min orgy in a sparsely furnished living room.
Somehow Kirk would get 'name' actors to appear in his schlock, but for quintessential examples of the worst of SF made one day wonders, look no further than Mr. Kirk's tedious filmography.
Written by Vinegar Syndrome's own Joe Rubin