District Loading Glitch
From Halopedia, the Halo wiki
The District Loading Glitch allows players to access partially loaded districts on Mombasa Streets in Halo 3: ODST. The main use of this glitch is entering the normally inaccessible areas of the half-loaded district.
Start at the district you want to glitch. Enter another district by opening and going through a large gate. Now locate an entrance to a third district. Get close to the gate leading there, but do not go into the illuminated gateway as this will trigger a "Loading... done", fully unloading the first district. At some point near the gate, the first district will partially unload and the third one will partially load.
You can see this happening when the game lags for a moment; this is why the glitch is also called the "bump glitch". This may be difficult to notice, but it will happen relatively far away from the gate, meaning that you will certainly trigger the partial loading by walking to a few meters away from the gateway. In co-op, the partial load is very easy to notice, as any teammates left behind on the first district will teleport near you.
After the partial loading is triggered, return to the first district without triggering the third district's "Loading... done" message. The first district will now be glitched.
Effects and uses
A partially loaded district contains almost all geometry, as well as all the enemies that would normally be there. Disappearing objects include:
The doors' disappearance can be used to access normally closed-off areas. For example, the doors leading to a normally inaccessible plaza in the northeast corner of district 8 (just south of Tayari Plaza) are open. Because viewing an Engineer-created glyph in the nearby district requires the player to cross the partial load trigger, some wrongly think that viewing the glyph makes the doors disappear. A larger area accessible with this glitch is the southernmost tip of the map. In the room with an audio log inside, there are multiple normally locked security doors that will be removed by the glitch, opening the way into the area behind the Energy Barriers. There's also an equivalent room in the Kikowani Station district, allowing access to the unmapped area there.
The data for a Halo level is stored in units called "BSPs". Each district of Mombasa Streets is a separate BSP. Halo 3: ODST has two BSP Memory Areas (BMAs), where loaded BSP data is stored. The first is the current BMA, used to hold data for the district the player is currently in. The second is the adjacent BMA, which stores data for the district a player is about to enter.
When a player approaches a district boundary, the district they are approaching is partially loaded. Incomplete pieces of data for the district being approached are copied into the adjacent BMA, replacing any data that was previously there. When the player actually enters the district, the remaining data is also loaded, and then the BMAs are swapped; the exited district becomes the adjacent, and the entered district becomes the current.
The glitch, then, stems from passing between loading boundaries in such a manner as to cause partial loading to fail. The district one chooses to glitch has its data partially overwritten by data from another district; that data is then restored later, but incorrectly so.