On the last mission of halo 2 the wraith at the end right before the scarab is about to shoot open the door is always flipped over with a brute standing on top. I assume this is a bug but it has been like that since mcc came out so im not sure.
In the beginning of the ark in halo 3, If you look at the soldiers on the pelican before you drop their faces look white as bone in places. Always thought this was a weird lighting issue that almost makes them look like skeletons.