The Bump

The Bump is a strange phenomenon that occurs in Halo 3 when you walk close to an object that you can interact with such as vehicles, weapons, or crates. Most players do not notice this bump because they are usually more focused on game such as getting kills or looking for the flag. However, players who spend a lot of time in Forge will surely notice the bump. Usually the bump doesn't cause objects to collapse or leave their current position, but in cases of extreme lag, the bump can cause objects to perform in a strange way. In the case of the Elephant, the bump, under extreme lag, will cause the Elephant to start shaking at first, and then after a few more bumps, the Elephant will violently flip over often crushing the players that are on the side of it. It is unknown why this bump occurs. It does not occur with unmovable objects such as walls and shield doors. It also doesn't occur in the Campaign mode with any kind of object or vehicle.

Video
In one of Rooster Teeth's behind the scenes video for Red vs. Blue, they talk about the bump and ways to get around it when filming machinimas.

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Trivia

 * The bump is exclusive to Halo 3 and has never happened in any of the previous Halo games or Halo Wars.
 * The bump appears to reset every few seconds after it has occured.
 * It is unknown whether the bump was meant to be in the game or if it's a glitch.