The YF-16 made its first unintentional flight at Edwards AFB, CA in 1974.
There’s an article explaining this event at this LINK
Nothing more than a high-speed taxi was planned for that day at Edwards AFB. Oestricher (the test pilot that day), who was to split flights in the initial test program with fellow GD pilot Anderson, had a run card to taxi the airplane up to 135 knots and then bring it safely to a halt after checking out the pre-takeoff handling characteristics. The first flight wasn’t scheduled to occur until two weeks later on 2 February.
From here the tale tends to differ, depending on who’s telling it and which version was heard. This much is fact: The high-speed taxi turned into a wild series of roll oscillations that ultimately caused the right horizontal stabilizer to strike the runway, at which point Oestricher took the airplane up to avoid wrecking it.
Thus, the high-speed taxi run became a highly unexpected first flight, and the YF-16 joined the ranks of real airplanes that have actually gotten off the ground, versus the legions of those that exist in the minds of engineers or as concepts on blueprint paper (in computer memory, nowadays). And Oestricher became the Man of the Day for saving GD’s best hope for the future, the first airplane of a program that would bring the company one of the longest periods of employment growth in the up-and-down history of the aerospace industry.
Here’s what they consider the actual first flight. I was so inspired by this “little” jet that I wanted to be a fighter pilot and fly this F-16…man it looked sweet.