The parallaxed space textures make an ordinary sector become lethal.
There are two special textures called
MOONSKY1 (#80) and
BIGORBIT1 (#84), which are used to simulate an outer-space atmosphere. If either of these textures are parallaxed on the ceiling or floor of a sector, they will kill any player who enters that sector. If you want to nullify the death effect, give the ceiling a palette value of anything greater than 0 (the floor palette has no effect, even if the texture is only parallaxed on the floor!). A palette value of 3 works great because it does not actually change the texture's colour.
Note: These textures will disrupt any other parallaxed texture in the map, check "References - FAQ" for more information.