To explain a bit more, this was actually a very rare glitch (which you can only really reproduce if you wipe everything [and even then, it still can be impossible to do]).
In Halo 3, if the player moves too slow in campaign (This event occurred at 8:43 [yes, I really took my sweet time]) then the next area will feature fewer enemies in order to help speed up the story. However, if you start blazing through, then the amount of enemies increases (this is seen on the crow's nest level). Also, if you take longer, the amount of time before the next checkpoint increases (for many reasons).
What happened here was after ripping apart several enemies, the game increased the number of baddies, but made one of them faster (the brute that runs into me). As soon as I rushed out of the loading room and right as the brute was charging, the checkpoint triggered (this checkpoint should trigger when you go through the door) and the game saved right before I died (.02 of a second [impossible to escape]).
This caused a loop (which I understand, but by the second time, I would have thought that it would place me somewhere else). Obviously, it doesn't process this way. So I kept experiencing the same spawn into the same death over and over again.
Campaign spawning rarely has problems (this is just one of the rare events). SWAT matchmaking, on the other hand, has problems with spawns (I have several videos of this [and I would have had a really good one with me dieing 5 times in a row on SWAT because of crappy spawning, but I forgot to save and I doubt it's still there]). Various times I have spawned in a bullet (this is really annoying [luckily, the guy must have never seen that he killed me like that {he was sniping with a BR} as I haven't found a video of it on YT]).