If you watch the ending movie on level 6, when the camera pans on the tower "a few hundred yards from your position", where 343 Guilty Spark introduces himself, look down and to the right a few feet relative to 343 Guilty Spark's location and there is another monitor (or 343 Guilty Spark)! I highly doubt that this is a reflection, but I just wanted to point this out because I have seen it several times.

Also, occassionally, at the downed dropship on level 6, there are a couple of shotguns next to each other be the shotgun ammo and there is an assault rifle on the tree stump.

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