Duke Nukem: Critical Mass for PSP, a wholly different game than a version released on Nintendo DS in 2011, was discovered on an innocuous DVD-R.
Far from a piece of random archival material, Gibson found the actual source disc that would have been used for writing the game onto UMDs, the ill-fated proprietary media Sony used for retail PSP games. This led to further complications, though, as he had to find ways to access information locked inside of Sony's proprietary files. While Gibson hasn't found way to make the game fully playable at this point, he was able to access game code and view 3D models for not just the series' iconic Pig Cops but also Duke himself. Duke riding a jetpack, no less.
Gibson's complete report on the discovery is fascinating as both a document of a lost piece of lore in one of gaming's most infamous series, but also as a view inside the challenge of archiving gaming's history. Even with the discovery of the game, sharing the data hidden on that DVD-R remains a challenge due to the proprietary files within.
[Images: 3D Realms]