Bungie's admitted to Eurogamer that the Halo 2 E3 2003 presentation was a fanciful snapshot of a game that could never be recreated on Xbox hardware.
"The graphics engine that we showed at E3 2003, driving around the Earth city... That entire graphics engine had to be thrown away, because you could never ship a game on the Xbox with it," said Chris Butcher, engineering lead on Halo 2.
"Through putting ourselves through hell, we were able to do a five-minute demo of it, but after we came back from E3 we had to admit that this graphics engine was never going to work - it was never going to support the kind of environments that are really important for a Halo game."
"So we literally scrapped the entire graphics engine and started from scratch."
That E3 2003 Halo 2 reveal can still be found on YouTube, complete with audience whoops and cheers as dual-wielding and Covenant vehicle-stealing mechanics were shown.
But an overly ambitious E3 demonstration turned out to be the precursor of a wild development cycle, in which hype and disorganisation created a monster in need of a brutal haircut and weight-loss programme.
"We were building stuff that just couldn't be played, in any engine," revealed Butcher. "We built, and detailed, and went a huge way down the path with a whole bunch of environments and levels for the game that just totally didn't make it.
"If you look at the level with the Flood, inside the quarantine area - that is the remaining 20 per cent of a gargantuan, sprawling level that was meticulously built and hand-constructed, but that could never, ever have shipped in any engine."
Head over to Eurogamer's detailed, seven-page interview with Bungie about the making of Halo 2 to find out much more.