Continuity mistake: While playing on any level in Campaign mode, you may have one gun equipped, but a cut scene will always start with you holding an assault rifle.
Bug: When you get to the first downed life pod on the second level, if you park the warthog right at the top of the hill and get in the turret, when you look at the big structure, the landscape will change to a snow-covered hill that is a landscape from another level.