One of the most infamously delayed games of all-time (if not the most).
It took 3D Realms 10 years to release a screenshot of the game.
Although older media does exist, including several screenshots from 1999 and a video from 2001, those versions of the game are, obviously, outdated and have been significantly reworked since then.
At least, we assume they have. God, imagine if it came out now looking like a shiny 2007 game, but still played like something that predates Half-Life. Then again, it worked for Serious Sam, and he was pretty much a carbon copy of The Duke.
The game's abbreviation, DNF, is also used in racing to denote 'Did Not Finish'. 3DRealms are aware of the irony.
In 2009, 3D Realms went bankrupt, hopes of the game to come out were crushed. Then in 2010, Gearbox Software announced they would release DNF the following year... causing hell to freeze over while pigs flew in 2011. There was some backlash, of course, but just seeing this game on shelves was impressive.