Typical that I forgot the obvious example of "The Bowmen". Another which occurred to me while I was out today was "The Demon in the Cathedral", an article in FATE which claimed to be a true account of events which happened in Mexico City Cathedral, but which was actually a garbled version of "An Episode of Cathedral History". There's an old article by me about it in the Ghosts & Scholars Archive on the website.
And now I've remembered another example, which I'm kicking myself for forgetting as it derives from a story by an author I love with a vengeance - Jack Finney. Here's what I said on the subject a few years ago in the Everlasting Club:
I've been reading (and immediately rereading!) About Time, the collection of Jack Finney's lovely, yearning and evocative time travel stories, which combines the contents of his The Third Level (1957) and I Love Galesburg in the Springtime (1962: I first read it back in the sixties in a paperback edition with one of those characteristically sexist covers of the time - go and look it up on the Net if you want to know what I mean!). While hunting around on the Net for info on this and on Finney's two glorious time travel novels (Time and Again and From Time to Time), I came upon a fascinating essay (http://magonia.haaan.com/2010/rudolph-1/) about the appearance of a man in Victorian clothing in the middle of Times Square, New York, in 1950; he apparently looked round in confusion and was immediately knocked down and killed by the traffic. He was identified as Rudolph Fentz, who went missing in 1876 after going out for a stroll and a smoke before bed. The account is retold in books and websites about true mysteries and only relatively recently has it been realised that it derives from a fictional source: it's one of the peculiar events which constitute Jack Finney's tale, "I'm Scared" (from The Third Level), and there's no trace of it prior to this (or, at least, prior to the first publication of "I'm Scared" in Colliers in 1951 - the date of Fentz's reappearance in New York is updated to 1955 for About Time).
Ambrose Bierce, Jack Finney, M. R. J., G.N.S. It's not as though the source authors are obscure, even if, in the cases of Bierce and Finney, their stories are perhaps lesser known than their respective greatest hits. Cliff 'The Orson Welles of Salford' Twemlow's mighty The Pike is perhaps of more specialist appeal, though it sold well enough for a film to be made (and duly shelved). Never read it, don't have a copy, but seem to recall that Harlan Ellison's Croatoan sparked, or was said to have sparked, the "giant alligators in the New York sewers" scare (or is that urban myth on top of urban myth?)
Not yet been able to psyche myself up to retrieve it, but stuffed up top of a presently inaccessible (as in obscured behind a bookcase) cupboard, I've a box packed solid with clippings & zeroxes. Somewhere amongst them, a two page newspaper item concerning a couple who believe their house to be haunted. The reporter who interviews them is sympathetic, they seem genuinely spooked about something (i.e., they don't come across as rabid attention seekers), but at close of the article, a caveat. The journo has since mentioned the story to a mate who recognises striking similarities between their story and the film version of The Amytyville Horror, most notably the scene where Kathy glimpses her daughter's pig-faced imaginary friend glaring daggers at her from the girl's bedroom window.
I Love Galesburg in the Springtime (1962: I first read it back in the sixties in a paperback edition with one of those characteristically sexist covers of the time - go and look it up on the Net if you want to know what I mean!)
The (low-strength) X-ray Specs one? No surprise I recognise it from one of Mr. Marriott's House of Fanatic publications (we so need an index).
From the first, I set myself against "literature"; the story was the thing, and no amount of style could persuade me to select a story that lacked genuine, unadulterated horror. For those who wanted something high-brow there was plenty.