Game Grumps - unreleased videos (lost content of Let's Play YouTube channel; 2012-2015)
The Game Grumps are a Let's Play YouTube channel, hosted by Arin (Egoraptor), Jon (JonTron) and later Dan (Danny Sexbang). Starting in 2012, they now have a strong fanbase with almost 4 million subscribers total. Unfortunately, some of their material is unreleased, half-recorded or completely lost due to various reasons.
Conker's Bad Fur Day
A few episodes (three according to Arin) of Arin and Jon playing Conker's Bad Fur Day were made, however, were never released and the let's play was discontinued. As explained in the first 'Banjo-Kazooie' episode and the panel Arin did at Shadocan, Arin thought the game was bad while Jon highly disagreed, "but was slowly realizing why", so they weren't talking and had a really awkward dynamic. On top of that, it was a dialogue-heavy game that started off really slow, so it was really hard to put funny dialogue on top of it which was another reason Arin hated it: a dialogue-heavy game wasn't really the best choice for Game Grumps. Arin called the experience depressing and called Conker the worst game they had to play on the show.
A similar incident happened with the initial playthrough of Demon's Crest. Jon wasn't having a good day, Arin liked the game but Jon hated it and they actually got into an argument about it ("Blood was shed" as Arin put it.). The episode also lacked a lot of the humor the channel is known for. They later replayed it on the show from the beginning, to which Jon mentioned that when they first tried it he "didn't much like it", so he was giving it another chance and that he originally didn't approach the game with an "old-school mindset". It has been mentioned by Arin in his aforementioned Shadocan 2012 Panel and their Yoshi's Island playthrough.
And as confirmed at Magfest '11 there was an episode for Dino City that was made but never released. Since Jon had already reviewed the game on his channel, the commentary consisted of (possibly only) referencing his review.
The Legend of Zelda: Link to the Past/Mega Man 7
Some games like The Legend of Zelda: Link to the Past, Mega Man 7, and possibly others, were replayed and re-recorded as their first playthroughs were trial runs for the show to test out recording and things like facecam.
Mario Party 4
As revealed in episode #2 of Yoshi's Island, there was a planned two-hour session of Mario Party 4. They recorded the audio but forgot to actually record the gameplay footage. They have stated that they may release the audio in the future, but this has not happened yet and seems a little less feasible now since Jon has left the show. The game ended up being re-played anyways on one of the channel's spin-off shows 'Steam Train'.
As revealed in episode #67 of Sonic '06, they recorded themselves playing Super Metroid but lost the footage as it was contained on Arin's computer which later died on them. They did want to play it again for the show and as revealed in episode #5 of Journey to Silius, it was attempted, but the footage ended up being lost again.
Some footage of Sonic '06 was lost partway through so they had to start their playthrough all over again.
Nickelodeon Guts/Castle Crashers
Nickelodeon Guts also had to be replayed because of the original playthrough's capture failing and four hours of Castle Crashers audio was lost because of another computer malfunction of Arin's laptop getting a blue screen of death, which was used for recording the audio.
Sonic Adventure 2/Strider 2
Two unknown cases are the original playthrough of Strider 2 and a possible Sonic Adventure 2 episode.
As stated by Arin on Twitter, after he said he hated the Sonic Adventure series with a lot of controversy following it, he replayed Sonic Adventure 2 with Jon, who said it wasn't as great as he remembered. But it's unverified as to whether or not they were recording that playthrough for an episode of "Game Grumps". Especially since there was a similar fast-forwarded event during one of the episodes of Sonic '06, in which after they had discussed the game's quality earlier, Arin showed Jon Sonic Adventure 2 on the Dreamcast to show that the gameplay was similar, to which Jon agreed on.
With Strider 2, they had lost the footage and tried to salvage it, to which their editor Barry said it was unsalvageable. However, they make no reference to an original playthrough in the version they did end up uploading. It's unknown if the footage did end up appearing in the currently uploaded playthrough or not.
Sonic Adventure DX
There is one episode of the Grumps' Sonic Adventure DX playthrough that was removed following some considerable controversy. During part 32, Arin and Dan repeatedly mocked the ambiguity of an online walkthrough for the game, particularly in regards to the Knuckles levels. During these levels, Knuckles is required to find three randomly scattered emerald shards in order to advance; as the shards' locations are not predetermined, the walkthrough simply provided performance hints rather than concise directions. Because of this, the Grumps admonished the walkthrough throughout their run of Knuckles's campaign, eventually culminating in Danny reading aloud the name of the walkthrough's author, Daniel "The Leadfoot" Rosmus. In the original upload of this part, Rosmus's name remained uncensored, prompting Game Grumps fans to seek out and harass him despite the walkthrough having been written during his middle school years. This caused considerable amounts of backlash, resulting in Danny providing an official apology for the incident on Reddit and Kevin Abernathy (Barry's replacement as editor following his retirement from the position) re-uploading part 32 with the name-dropping audio replaced with a voice saying "pizza" between two bleeps. Though Rosmus responded to Danny's apology by revealing that the harassment did not affect him, the uncensored video remains deleted out of courtesy.
Initially, there was a YouTube account that provided mirror uploads for the initial releases of every Game Grumps video, including the uncensored version of part 32. However, the account (and by extension, the video) has disappeared, having either been terminated by YouTube's staff or closed by the channel's owner. It is unknown if anyone is in possession of a copy of the video.
UPDATE: On December 9, 2013, Arin and Dan have started playing Super Metroid again on the show; they completed the game on January 14, 2014.
UPDATE: On September 27, 2015, a mega-compilation of the entire Sonic Adventure DX playthrough was uploaded to YouTube, containing the uncensored name-dropping of Daniel Rosmus at around 1:16:10. Special thanks to Azumi for pointing this out in the comments! It is unknown if the original uploader still has an individual uncensored copy of episode 32, though.