The Pentagon, the headquarters of the United States Department of Defense, is a five-sided building in Arlington County, Virginia that opened in 1943. The Pentagon has been nicknamed “Fort Fumble” since at least 1965-1966, when critics complained of its “fumbling” of the Vietnam conflict.
 
The Pentagon has also been popularly nicknamed the “Puzzle Palace,” cited in print since the early 1950s.
 
 
Wikipedia: The Pentagon
The Pentagon is the headquarters of the United States Department of Defense, located in Arlington County, Virginia. As a symbol of the U.S. military, “the Pentagon” is often used metonymically to refer to the Department of Defense rather than the building itself.

Designed by the American architect George Bergstrom (1876–1955), and built by Philadelphia, Pennsylvania, general contractor John McShain, the building was dedicated on January 15, 1943, after ground was broken for construction on September 11, 1941. General Brehon Somervell provided the major motive power behind the project; Colonel Leslie Groves was responsible for overseeing the project for the Army.
 
The Pentagon is the world’s largest office building by floor area, with about 6,500,000 sq ft (604,000 m2), of which 3,700,000 sq ft (344,000 m2) are used as offices. Approximately 23,000 military and civilian employees and about 3,000 non-defense support personnel work in the Pentagon. It has five sides, five floors above ground, two basement levels, and five ring corridors per floor with a total of 17.5 mi (28.2 km) of corridors.
 
Google Books
U.S. News & World Report
Volume 59
1965
Pg. 33:
The current favorite term in Saigon for the Pentagon is “Fort Fumble.”
   
8 November 1966, Billings (MT) Gazette, “Nothing New to Reporrt” by Addison Bragg, pg. 1, col. 5:
Every year someone comes up with a new name for The Pentagon—and every year’s new name proves more descriptive than the last.
 
It’ll take some doing, though, to beat the current label inhabitants of the five-sided paper mill on the Potomac have adopted for general use.
 
They call it “Fort Fumble.”
 
Google Books
Army Digest
1968
Pg. 57:
After it was built it was variously dubbed a Squirrel Cage, a Puzzle Palace, a Cement Sanitorium, the Pandemonium Palace on the Potomac, Fort Fumble and, most recently, Disneyland East and the Five-Sided Wigwam.
 
Google News Archive
1 June 1970, Pittsburgh (PA) Post-Gazette, “Little Fort Fumble,” pg. 10, col. 1:
NEVER MIND tight budgets, inflation and soaring war costs, the generals over at Ft. Fumble, as some Washington pundits call the Pentagon, decided that they needed a mini-Pentagon to house some 10,000 Defense Department employes working in rented offices spread through the District of Columbia, Maryland and Virginia. Officials estimated cost of the Minigon: $133 million.
 
Google News Archive
5 September 1971, Lewiston (ME) Morning Tribune, “The Military Mind Is At It Again,” pg. 4, cols. 1-2:
One of the namedby which the Pentagon is known in Washington is"Fort Fumble.” It’s funny because it’s true.
 
The truth derives as much from small, annoying puzzlements as it does from major issues.
(...)
It is apparent that mere civilian minds can’t comprehend such military urgencies. The best they can do is call the Pentagon “Fort Fumble.”—Oregon Journal.
   
23 August 1972, New Orleans (LA) Times-Picayune, sec. 1, pg. 17, col. 6:
PENTAGON HOLDS
MANY NICKNAMES
The Pentagon has many nicknames, which include: The Cement Sanitorium, the Squirrel Cage, the Puzzle Palace, Fort Fumble, Disneyland East and the Pandemonium Palace on the Potomac.

Google Books
Murder at the Pentagon
By Margaret Truman
New York, NY: Fawcett Crest
1993
Pg. 7:
“Well, welcome to the Puzzle Palace.”
“Thank you.” The classic nickname for America’s center of national defense, the Pentagon, was one of several. Fort Fumble, the Fudge Factory, the Five-Sided Wailing Wall, were used less frequently.
 
Google Books
The Silent Service.
Los Angeles Class
By H. Jay Riker
New York, NY: Avon Books
2001
Pg. 34:
Most simply called it “the Building,” but those in a more critical mood referred to it as “the Fudge Factory,” “Fort Fumble,” “the Squirrel Cage,” or — Gordon’s favorite by far — “the Five-Sided Wailing Wall.” “The Puzzle Palace” was a pet name contested by the occupants of the National Security Agency’s huge facility at Fort Meade, Maryland.
   
Google Books
Victory On The Potomac:
The Goldwater-Nichols Act Unifies The Pentagon
By James R. Locher
College Station, TX: Texas A&M University Press
2004
Pg. 88:
Frustrations with this horrendous process sparked fitting nicknames for the Pentagon:  Five-Sided Squirrel Cage, Potomac Puzzle Palace, Disneyland East, Concrete Carousel. When the Defense Department performed poorly, many referred to the Pentagon as Fort Fumble or Malfunction Junction.