Giant Spartan Glitch
From Halopedia, the Halo wiki
The Giant Spartan Glitch was a glitch in Halo: Reach which caused the player's unique Spartan model to suddenly grow to six times their usual height. The glitch was caused by lag due to Bungie's servers being overloaded with players on Xbox Live during the first few months of Reach's release. The glitch is no longer reproducible.
When triggered, the Spartan immediately grew to 6 times the typical size, making the player roughly 41 feet tall. While their massive size carried over to their hitbox, making the player a larger target, it did 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 take huge strides while covering minimal ground, as though they were running on ice, since they were still moving at the same speed players normally run at. Bullets appeared to fly out of their knees, since they were still being fired at the same height the player would normally stand. The player's current weapon still appeared in their hands, however, it did not increase in size.
It is unknown if the glitch was possible with the use of the Elite multiplayer model.