Sudden death of ("invincible") player in Doom II level 30,
Icon of Sin:

The normal safeguards against "telefragging" by monsters
are disabled on map 30 of various Doom II-based games (PC
Doom II, PC Final Doom, Xbox Doom II, etc.).  If a monster
is "spawned" into/onto a player, or a monster follows a
player through a teleporter on this map level, the player
can be destroyed, even when "invincible" or "invulnerable".

In the Icon of Sin map, the Boss entity will release an
endless series of flying brown "skull cubes".  When a cube
reaches its destination, a monster is teleported to those
coordinates.  Any living thing sharing the vertical column
of space at those X and Y coordinates will be destroyed.

PC Doom 2 players can access a demo showing how skull cubes
can destroy monsters and players, by clicking here.