Forgotten, But Not Lost: The Story of F-4023, the F-1 Engine that Never Flew

"Engine That Almost Launched Apollo 8 Moonshot Found 40 Years Later"
Space.com (December 22, 2008)

"Five seconds before the first men left Earth for the Moon on Dec. 21, 1968, the rocket they were riding ignited its first stage's five engines. They were the first astronauts to launch atop the Saturn V, and that day 40 years ago at 7:51 a.m. EST was the first time those particular five F-1 engines had fired together...."

One of the five F-1 engines shipped to the Cape for Apollo 8 had a leaky pump, and had to be replaced. This article tells how the F-1 with serial number F-4023 got sent back for repairs, and forgotten for 40 years.

More, at "Rocket engine that almost launched Apollo 8 to the Moon found at NASA 40 years later" (CollectSpace (December 22, 2008)).