I've noticed some issues with single use teleporters. It has happend in Doom and Heretic, no idea how Hexen teleports behave.
1) Any Missile object pass through the line from either side: the line was used and no longer works for players or monsters.
2) Some other object pass through the line from the backside: the line was used...
In both cases there is no teleportation, just the line becomes used. Single use doors, etc seem to work even if missile objects have been shot through them.