Never Recycle a Building: The Amusement Park of Doom from the climax was the Real Life abandoned ruin Saltair, a former Mormon dance hall and bathing pier; Herk Harvey happened to run across it while driving across Utah, derelict and decrepit but yet never torn down or sold, and was so inspired by its creepiness as to film it as the centerpiece.
No Budget: Made for $30,000 ($250,000 in today's money), but since the creative team had lots of experience making low-budget industrial and educational films, they knew how to shoot resourcefully and the film is quite professionally done.
One-Book Author: Herk Harvey never directed another feature (although he did helm many educational shorts).
One-Hit Wonder: Herk Harvey's industrial short production company made only one narrative film, this one, and never did one again.
Unintentional Period Piece: The Utah-filmed scenes act as a time capsule, because the locations have changed so much since the early 60s. Saltair burned down in 1970 (a near-replica of the pavilion was built nearby about a decade later and is used as a concert venue now). The places in downtown Salt Lake City used in the film look quite different now.
About eight minutes of footage belonging to a sequence set just before the final danse macabre was lost in processing when the film was overexposed. It would've shown the ghouls appearing from behind the dock pylons at the edge of the water, making their way to the pavilion.
Candace Hilligoss (who played Mary in the original) had been in talks to greenlight a sequel, and had even written a treatment and screenplay. For good or ill, the deal fell through. Her claim was that Peter Soby feigned an interest in her script just to get his foot in the door with Harvey and get the film rights to do a remake. In acquiring these rights, Hilligoss wound up being written out of the deal and Soby went on to produce the 1998 remake (which had nothing to do with Hilligoss's script).
Hilligoss was asked to make a cameo in the remake, but turned it down do to feelings of betrayal and disrespect due to the above reasons.