Introduction:

I am bad at modding and patching Quake-style 3D games, and with Kiss Psycho Circus (KPC) being one of my favorites in the genre, there have been a handful of glitches that I've experienced in the game playing it on & off for decades. Most of them I discovered either on my first playthrough as a kid, by accident, or by experimentation. They are progressing-ruining glitches that rookies of KPC can make.

If someone were to make a "definitive edition" or "patched version" of KPC, which I don't have the means to, these are all of the areas that have problems that I know of.

The Dreamcast (DC) port of KPC if I can remember ran off a downgraded version of LithTech 1.0 (Blood 2's engine) instead of LithTech 1.5. With the DC version likely using LithTech 1.0, the DC has a lot more skippable glitches the PC doesn't have like shimmy-ing up certain walls. Pretty much any glitch Blood 2 has, KPC on DC also has in my experience. With that, I'm focusing solely on the PC version. The PC has about half the amount of skippable glitches the DC has.

The DC also has cancelled levels due to system limitations, which are Freak Show 1 (R1M4a), the inside of Morbid Manor (R2M2b and R2M2c), Molten Vein 2 (R4M2b), Gloomvale (R4M4a), and the intro level of the Nightmare Realm (R5M1a). Out of all the boss fights, the DC only retained the Nightmare Realm boss fight (R5M1b). 

Those Morbid Manor levels have Peter's Belt and Vest, which are not skippable. Molten Vein 2 has Gene's Vest, and Gloomvale has Gene's mask. Those are not skippable either.

PC glitches with descriptions and pictures:
 

R1M1b (Bad Streets 2): This shed's interior has a key on top of a crate. You're supposed to get the Boots to jump high enough onto the crate, but it is very easy to jump on top of a Stump dog's head to get on top of the crate. I'd suggest making this a room in the shed notablly bigger, with the key being placed on top of a second height crate.

This is something that a rookie player might try to do, which will make the rest of the Water Realm extremely hard without the Boots. The Water Realm has a number of crates that need to jumped onto with the Boots.

R1M3a (Cathedral 1): These stairs really need to be fixed. Most of the time, the player gets stuck in the geometry of these stairs. The only fix is to load a previous savegame, ruining the player's progress.

R2M1b (Earth Rift 2): It is quite easy to skip the Boots by standing on the edge of the slab that the Boots are on, and then jump unto the upper platform. I think moving the slab that the Boots are on should be pushed further away from the upper platform, so that way, the player (without Boots) won't be able to jump onto the upper platform. It will force the to pick up the boots.

Without the Boots, the player will have to complete the Earth Realm in a very glitchy way, making everything hard.

R2M3a (Unholy Ground 1): It is also easy to skip the Plate by standing far enough from the lever, pressing it to go to the next area without the Plate. The game expects you to pick up the Plate first, and then press the lever. I think perhaps put the plate in front of the steel barred gate, with a strict trigger that is wide, as high as the sky (highlighted).

R3M1a (Waste Wharf 1): You can completely skip the Gauntlets & Boots by going straight to this part where the key is, and use the monsters to jump on their heads. I'd suggest re-designing this area so that the Boots are required to jump high enough. Also, re-design it so that jumping on monsters's heads won't let you jump high enough without boots.

R3M2b (Poison Plant 2): Skipping the Vest is very easy. Simply jump on the monsters's heads, and then jump over the Vest. To prevent this, I'd create a trigger barrier over the entire highligted area (right up to the ceiling).

R3M3b (Acropolis 2): Skipping the Plate is also easy. Simply jump on the monsters's heads, and then jump over the Plate. To prevent this, I'd also create a trigger barrier over the entire squared area (right up to the ceiling).

R3M3b (Acropolis 2): It is rather easy for the player to forget picking up the whip (the Scourge) in the early part of the Air Realm. Right in the hallway where there is a mandatory grappling hook in a ceiling hole, there is a Shotgun (the Magma Cannon) you can pick up right before the hook.

I think in this area, there should be a whip instead of a shotgun, so that way, the player can progress. One time I casually played the game, I forgot to pick up the whip early on, and I couldn't progress any further. I was stuck in this hallway.

R4M1a (Doom Forge 1): The ledge (circled) is somewhat easy to jump onto with enough practice. This makes skipping the Gauntlets possible, which will stop your progress later in the game. I would recommend redesigning this room so the ledge is blocked.

R4M1b (Doom Forge 2): I think this part (circled) should be an entire wall that continues to the top of the ceiling, so that a tiny ledge that squshes you doesn't kill you when you ride up the lift. The lift, especially if you stand on it at a particular angle, can make the circled part kill you.

R4M2a (Molten Vein 1): The Belt is easily skippable if you jump around the red bridge. I would recommend placing the Belt directly in front of the opening in the wall (spot shown by the arrow), so that when you crouch down, you are forced to pick up the Belt no matter what. Widen its trigger area as well to make it unskippable in that spot.

R4M3a (NetherStation 1): The Plate is easily skippable if you jump on top of this piece of the ladder (arrowed), and then jump from that to the platform above (arrowed). Perhaps make the ladder approachable from the spot I illustrated?

Alternatively, the player can also jump on a monster's head and jump onto a crate, and then onto the upper platform. I would remove the crate entirely as it serves no purpose.

R5M1a and R5M1b (Nightmare 1 and 2): It was intended for the game that when the Nightmare Realm starts, the character you select will only have the items, weapons, ammo, armor pieces, etc he had after beating the Realm. Here is a scan from the strategy guide mentioning that, and this is a common assumption that I've seen players in live streams make. You can also see the difference in the strategy guide's screenshot with the inventory items showing.

If you click on my pictures below the guide scan (taken from my lastest playthrough of the game), you'll see that in the PC version when starting the Nightmare Realm all of the inventory items are taken away except the Crystal Ball, and all weapons are given with 200 ammo each. All armor pieces are also given. 

Likewise, when completing R5M1a, all items and ammo you collect there will also be taken away after entering R5M1b (the boss fight). All ammo will be reset back down to 200 ammo for each weapon.

This has frustrated a number of players that I've seen in live streams, who try to select the character that they finished with the most ammo, inventory items, etc.

I remember one selected Paul Stanley because when he played as Gene Simmons, he had very little ammo with the rocket launcher missing by the time he completed the Fire Realm. But when he selected Paul Stanley (where the streamer finished with a lot of ammo & all weapons), he was unhappy that all of Paul's ammo was shrunk down to 200 with his Rage Orbs all taken away. And when he grinded for ammo for R5M1b, he was mad that he lost the ammo he grinded, plus a Rage Orb he found.