The Bump is a strange phenomenon that occurs in Halo 3. It is a quirk in the physics engine that happens when a player walks close to a movable object. It causes the object(s) to suddenly shake or move, as if they had been nudged.
The Bump commonly goes unnoticed by players in multiplayer, who are typically more focused on the objective of whatever gametype they happen to be playing. On the other hand, players who spend a lot of time in Forge are almost guaranteed to have seen the Bump at least once.
The Bump is more likely to occur when movable objects are placed near each other, and it is extremely common when items are stacked on top of each other. Usually, the Bump doesn't cause objects to collapse or leave their current position, but in cases of extreme lag, the bump can cause some objects to perform in a strange way. The Elephant, when affected by the Bump under extreme lag, will start to shake at first, with additional Bumps eventually causing the Elephant to violently flip over, crushing any players that were standing next to it.
It is unknown why the Bump occurs, though it may be related to the closeness of the objects it affects. It does not occur with immovable objects such as walls and shield doors. It also doesn't occur in Campaign. The Bump resets every few seconds if a player stands still next to an object.