Giant Spartan glitch
From Halopedia, the Halo wiki
The Giant Spartan Glitch is a glitch in Halo: Reach that affecting Spartans in multiplayer. It causes the player's model to suddenly grow to 6 times their usual height. The glitch was the result of lag early on in Halo: Reach's release, and can no longer be reproduced.
Details
When triggered, the Spartan immediately grows to 6 times the typical size, making the player roughly 41 feet tall. While their massive size carries over to their hitbox, making the player a larger target, it does not carry over to their running and fighting abilities, making their animations for such actions appear heavily disproportionate as a result. For example, their running animation now made them appear to be taking huge strides while covering minimal ground, as though they are running on ice, since they are still moving at the same speed players normally run at. Bullets will appear to be flying out of their knees, since they are still being fired at the same height the player would normally stand. The player's current weapon will still appear their hands, however, it will have not increased in size.
The glitch was caused by lag due to Bungie's servers being overloaded with hundreds of players over Xbox Live during the first few months of Reach's release. As of 2011 the glitch is no longer reproducible.
It is unknown if the glitch was possible while playing as Elites.