Mass Effect 2/Unused Audio

Post-Horizon Squadmates Pre-Horizon
The recruitment missions in Mass Effect 2 are divided up into two groups: Garrus, Grunt, Jack, and Mordin have to be recruited before the Horizon storyline mission, while Legion, Samara, Tali, and Thane can only be recruited after Horizon. (Legion in particular is a special case as its recruitment mission is not only much later than the other squadmates', but also triggers the endgame.) However, by using a save editor on the PC and 360 versions of the game to use the squadmates earlier, you can hear dialogue from the post-Horizon squadmates on missions pre-Horizon. This indicates that it was possible at some point to be able to recruit the squadmates in any order, and the decision to split them into groups came relatively late, probably when the developers realized that the Xbox 360 version of the game would have to be split between two disks. The only exception is the Horizon mission itself, where only Tali has dialogue, indicating that the Horizon mission was created relatively late in development and/or Tali was not originally a member of the "post-Horizon" group.

Mordin's Recruitment Mission
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Mordin has extra dialogue when Tali is in the party.

Garrus' Recruitment Mission
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Garrus has extra dialogue if Shepard picks either Legion or Tali to stay with him during the Blood Pack assault.

Grunt's Recruitment Mission
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Jack's Recruitment Mission
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Tali on Horizon
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As mentioned earlier, Tali is the only post-Horizon squadmate to have recorded dialogue on Horizon. There's also dialogue for both Ashley and Kaidan specifically reacting to Tali (or Tali and Garrus) being in the party.

Legion on the Collector Ship
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The existence of these dialogue files indicates that the timeline of the game changed significantly over the game's development; in the final game, the completion of the Collector Ship mission triggers Legion's recruitment mission.

Conrad Verner, Paragon Style
KGHD160dy80 Perhaps the most infamous casualty of the ME1 -> ME2 save importer (it even gets a reference in Mass Effect 3), this scene should play if you used the Charm persuade option in the first game to convince Conrad to back off the heroics. However, an error with the save importer causes the save options to read both "Charmed Conrad" and "Intimidated Conrad;" the game defaults to the Renegade option and plays the "Intimidated Conrad" scene, leaving this unused. It can be fixed in the PC and 360 versions by using a save editor to remove the Renegade outcome.

Post-Council Security Check
LJduDSbJKtA In Mass Effect 2, you have to go to the Wards before meeting with the Council on the Presidium, but there's functioning audio still in the game for doing it the other way around, with different dialogue depending on whether or not Shepard was reinstated as a Spectre. It can be viewed in-game on the PC and 360 versions by using a save editor.

Notably, the cut dialogue mentions that Executor Pallin from the first game died during the Battle of the Citadel and was replaced by Chellick, the detective Shepard worked with in the "Citadel: Rita's Sister" sidequest. This was contradicted/corrected by the Mass Effect: Retribution tie-in comic, set after the events of ME2, which refers to the still-living Executor Pallin.