It's time we start organizing the list of suicide traps on the Marathon Story Page. Here's a general list of suicide traps that work in the full Marathon as far as I know of (ignoring the original M1A1 and the Marathon Demo):
Defend THIS! - If you botch the grenade hop to the Fighter alcoves in the bridge area, you can end up become trapped in the area below. There's also two others that Hamish simply refuses to tell us about. ;)
Couch Fishing - The end can become a suicide trap if you fail to hit all the switches and forget to leave the door to the final areas open. We're just lucky there's no Pattern Buffers in the final area!
The Rose - There's a few lifts on this level which you can get stuck in, much the same way as on Blaspheme Quarantine later on. You can see them here.
Smells Like Napalm, Tastes Like Chicken! - If you enter the secret Flamethrower room but fail to escape, you'll realize it's a complete suicide trap, thus the Durandal terminal at the bottom.
Blaspheme Quarantine - Two ways to get trapped on this level: first, you can open the door to the secret ammo closet in the elevator room and then stay in there until it closes, trapping you completely; second, if you return to the first elevator after going through the level, you'll find it deactivated - if it's deactivated so that you can't enter the bottom door or head to the top floor, you can jump in the top and find yourself in a suicide trap, as shown here and in the Rampancy video for this level.
Bob-B-Q - As mentioned here, if you turn off the walkway platform and then fall off on the wrong side, you'll become stuck irrevocably.
Shake Before Using... - If you jump on the ambush platforms in the first bridge area as they are descending, you'll find it impossible to escape without killing yourself.
Colony Ship For Sale, Cheap... - You can become trapped in the corridors between the octagonal crusher rooms if you're not careful, especially if you go clockwise.
Thus far those are the only ones I know of, but I suppose we'll find any other ones as me and Martin continue our Tour of Duty.