Let's list the many ways Halo 2 wasn't the game Bungie intended to put out.
Grenade Jumping
A key component of every single Halo, Halo 2 had intended grenade jumps in multiplayer mpas. The problem? There was a bug that turned off physics in a game for everyone that wasn't the host. Meaning, in Halo 2, it was impossible to grenade jump in multiplayer LAN or LIVE, meaning intended routes were only possible for the host. People thought this was just a limitation of the original Xbox's processing power. Nope. Actually just a bug.
Warthog and Ghost Balance
The physics bug extended to the vehicles. Instead of being able to be flipped over with a skillful grenade like in Halo 1, 3, Reach, and 4, Halo 2 Warthogs and Ghosts will just flinch if being driven by someone that's not the host. This ruined the intended balance of these vehicles. The only way to reliably deal with them is the rocket launcher or a sticky.
When you play Halo 2 splitscreen, you'll suddenly notice how much more
sense vehicles make with the sandbox.
Havok Bug - Superbouncing
A bug in the version of Havok Halo 2 used allowed players to get on top of maps and structures they were never intended to reach. Seeing as how the maps were only balanced around the playable space, this allowed people to totally fuck over the intended map movement and weapon loops. People could take the bomb around the top of Burial Mounds via it's skybox, they could get on top of Zanzibar's defense base with the windowshades destroyed, they could get on top of the Windmill and be able to snipe the beach and into the base un-impeded. The only two maps you couldn't superbounce on were Tombstone and Desolation, and that's because they came out well after superbouncing was a known thing. And their fix for it was to strap giant fall zones over the tops of the maps.
Havok Bug - Super Ejection
If you had a teammate cram themselves into certain parts of a map, you could ram a Warthog into them and Havok would eject them at 100mph across the map. You could launch a bomb carrier from the beach of Zanzibar right into the base, or launch someone across Coagulation. Amongst other maps with similar spots. Ruins intended map routes and weapon loops, AGAIN.
Fall Damage
Halo 2 was intended to have fall damage. They had to remove it later in development to get the Sword to work without killing the user. Look at maps like Collosus and tell me they weren't balanced around fall damage being present.
BXR, Doubleshot, and other button combos
I'm just going to link my post on this
Vehicle glitches
Containment was intended to heavily feature Banshee fighting and Banshee battles. The middle structures of the map are entirely intended to balance the banshee, since they limit it's firing lines and give it something to dive behind to save itself from rockets. Unfortunately, a bug in how Halo 2's walls work means the Banshee was able to fly right into the map's skybox. Bungie had to just delete the banshee from the map in matchmaking, leaving half the base's intended action space useless.
The defense team on Terminal was supposed to have a Spectre outside the base to defend with. Unfortunately, a bug in the game made it so the Spectre would explode when the game started. The vehicle slot had to be turned off.
Map Escapes
The defense team on Headlong was supposed to have a banshee. It had to be removed in matchmaking because it allowed a team to lift the flag or bomb carrier out of the map.
It was possible to easily throw the bomb under Colossus, Coagulation, Ivory Tower, and many other maps. And there was no way to prevent it or any special vehicle or weapon configurations to do it, all you had to do was just throw it at the right time at a wall, and down it'd go. This pretty much let you instantly win neutral bomb games, as the other team could not get the bomb back.
Certain Affinity made Tombstone have default BRs, which meant that when Bungie had to update matchmaking, they had to force every gametype to have SMG starts for gametypes they didn't want BRs in, which meant Covy maps lost their PR start, breaking the theme because an outside developer decided to be cute instead of letting the gametype choose the starting weapon.
Bugs that were later fixed
Dummy glitched elites that allowed you to create invisible, invincible players that could pick up objectives.
Being able to ignore closed teleporters.
Being able to grab flags through 3 foot thick walls.
Being able to launch a flag across a map in 5 seconds by simply throwing it onto the right part of the map.
More time was spent playing Halo 2's glitches than the actual sandbox of Halo 2.