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Special Xbox 360 Cooperative Note:
When we were developing the Xbox LIVE Arcade version of Duke 3D, one of the things we looked at in detail was co-operative play. More specifically, whether or not the original level construction would leave you stuck with no way out. We spent a lot of time looking for this, and found only one place where this was possible. It was in Critical Mass with the spiral stairs shown here. During the normal single player path, going up the spiral staircase would cause them (and the room at the top) to collapse, leaving you with no way to get back. This is a non issue in single player, but in Co-Op play, we discovered a problem.
In Co-Op, if you got past the collapsing stairs, and then died, you would get taken back to the start of the level. The problem with that is that now the stairs are already collapsed, and you were stuck - there was no way to get past the stairs in Co-Op.
We examined the map data, and discovered that in Dukematch play, this map had special map data so that when the stairs were down, there was a way around. In Dukematch, there is a switch on the wall that was not there in single player or Co-Op originally which would open up a teleport and let you bypass the collapsed stairs. The decision was made to alter the original map data for this instance. We changed the map so that when you played Co-Op, this teleport was also available to you. It is NOT available in single player, and this is the only instance in the entire game when we went back and changed existing map structure for the Xbox LIVE Arcade version. Otherwise you'd get stuck.

I took a look into this. Once you step out of the staircase, it closes a door in front of the staircase, and the ceiling starts to fall. And when you step in the next room with red lights, it opens a teleport in the area before the staircase for co-operative.
The problems comes that there are two triggers to these events, so if you die after you actived the first, you will be locked. You can also see this in single player, by stepping out of the staircase and instead of entering the next room you go back using Steroids.
I think that to fix this all you need to do is move the TOUCHPLATE with lo-tag 5 to the same sector as the one with lo-tag 8761.