For the Halo 3:ODST achievement "Good Samaritan", if you mess up can you

[P.J.]

just replay the level? I was between the missions "ONI Alpha" and "Kizingo Boulevard" and the little progress bar for "Naughty Naughty" showed up, saying I killed an engineer. Am I now screwed over for the rest of the campaign... or can I just restart that level and try again?

 

[TRIPLEM]

No - you lose 'the game' for ever.

 

[Killer Thriller]

Well, you would have to reset the entire game progress saved and refile your saved folders afterword...