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Of American-English origin, the colloquial phrase to shoot oneself in the foot means: to act in a way that inadvertently damages one’s cause or reputation or spoils one’s chances.

There have been innumerable literal uses of the phrase to shoot oneself in the foot referring to accidental shooting. These are two early examples:

1-: From The California News, published in the Buffalo Commercial Advertiser (Buffalo, New York, USA) of Friday 15^th October 1852 [page 2, column 4]:

A bloody rencontre took place at Los Angeles, between Mr. A. Russel [sic], of the Sacramento Union and Mr. W. A. Cornwall, private Secretary of Governor Bigler. It originated from a publication in the Star, reflecting upon Governor Bigler. Mr. Cornwall attacked Mr. Russell with a bowie knife, wounding him in the face and the back of the head. Mr. Cornwall also accidently shot himself in the foot.

2-: From City Intelligence, published in The Daily Picayune (New Orleans, Louisiana, USA) of Tuesday 24^th May 1853 [page 1, column 7]:

Aᴄᴄɪᴅᴇɴᴛ.—Mr. Frederick Bruno, while incautiously handling a pistol, at the lake end of the Pontchartrain Railroad yesterday, accidentally shot himself in the foot. He was sent to the Charity Hospital.

These are the earliest figurative uses of the phrase to shoot oneself in the foot that I have found:

1-: From an interview of Admiral William Morrow Fechteler (1896-1967), who had just been appointed Chief of Naval Operations—interview dated Norfolk, Virginia, Sunday 12^th August 1951, by Harry Nash, Associated Press staff correspondent, published in several newspapers on Monday 13^th August 1951—for example in the Buffalo Evening News (Buffalo, New York, USA) [page 1, column 4]:

A smile spread over the face that bears the stamp of 22 years spent at sea as Admiral Fechteler recalled:
“The closest thing to a serious jam I ever had was back in the days when I was assistant gunnery officer aboard the battleship West Virginia.
“I had a five-inch battery and we were firing star shells to provide illumination for the range finders on the 12-inch guns.
“A star shell from one of my five-inch guns struck the muzzle of one of the 12-inch guns. You might say I shot myself in the foot,” Admiral Fechteler laughed.
The incident disclosed faulty design, which the Navy later corrected. “So I came out of that one all right, but my face was awfully red at the time,” the admiral said.

2-: From Mankind in the Making: The Story of Human Evolution (London: Martin Secker & Warburg Ltd, 1960), by the U.S. anthropologist William Howells (1908-2005) [chapter 6: Primates in the Past; page 97]—here, the phrase to shoot oneself in the foot is part of an extended metaphor comparing the art of writing to the use and care of guns:

AS THE ROYALTY of primates, we have in the last two chapters been looking around upon our vassals. (Here you have the editorial, the royal, and the human “we,” all rolled into one.) Nearest the throne we see the apes, the royal dukes; beyond them are the barons, the monkeys of the Old World and the New World, and beyond them in turn a clownish peasantry of prosimians. This little hierarchy seems to make a nice “family tree” for us, and much writing has been based on such a view. But the view is too simple, because it pays more attention to primates in the flesh and less to primates of the past, to whom I am now turning. Because of this distinction, certain common useful phrases can be dangerous and misleading, such as “family tree,” “ancestor,” “forerunner,” or “missing link.” Like guns, they will do the right thing in the right hands, but they are loaded, and ordinary citizens without Ph.D.’s are not the only ones who have accidents with them. Many a specialist has shot himself in the foot when he thought he was only cleaning a paragraph *.

* “Many a specialist has shot himself in the foot when he thought he was only cleaning a paragraph” probably alludes to shooting oneself in the foot when cleaning a gun.