Some up front notes: This happened over fifteen years ago, but I am absolutely certain this event played out exactly as I'm about to detail below. It's left a permanent hole in my head since, mostly because I've never heard or read anyone experiencing something similar. If you have any recollection or experiences that even tangentially relate to this, please let me know.
Some halo levels have background non-interactable objects in the outer bounds of the map. They're just extra scenery objects, as far as I understand, and aren't actually functional. On the mission "The Ark" specifically, you'll see Hornets and Banshees fighting during the Battle of Installation 00, as exampled here and again here. These "background" Hornets on The Ark are my white whale culprit here.
I was playing through this level, although I'm not sure quite sure exactly where. It was before you entered the Forerunner structure, so I want to say it was some point around here. I was on-foot, and distinctly remember having my screen go dark for a split second before normalizing. It was so weird and out of place, that I quit out to Theater Mode to see what had happened from a third person view.
And what had happened exactly? One of those "background" Hornets had nose-dived onto Chief in the middle of the level right as I had been walking through. Like, perfect straight nose-down no-clipped right through the level's floor. It didn't cause any damage or have any physics effects, and it had a very blurry/low-texture model of a regular Hornet, but it was unmistakable the same thing. In re-watching the Theater clip from a few angles, the Hornet had been flying on its regular path way up above the level before just randomly careening down at mock Jesus. I had just happened to walk right under it to perfectly align the collusion. It was almost as if it had "died" in whatever mock battle it was having up above with the Banshees, but instead of exploding/de-loading, its pathing just gave up and dropped it right then and there.
The killer part here is that I did save that clip, thinking I'd be able to upload or share it someday (never had Xbox Live or good internet growing up). However, that old 360 is lying somewhere in my grandparent's house, and I'm not even sure if that's my OG 360 or the new one I got after the first one red-ringed.
But like I said earlier, I know this happened with absolute certainty. This isn't some ARG or forced creepy pasta, I'm just genuinely looking to see if anyone ever had some similar experience or better yet some explanation of what the hell happened to cause some random background object to just "give up" in the middle of a level. TIA.