Multiplayer synchronization workarounds
Because of the limitations of netcode in the Gearbox port, map authors have developed workarounds to synchronize game state between server and clients. These workarounds often take the form of "biped crushers" or "script rooms" -- hidden areas of the map where special scripted objects like vehicles and bipeds are used to coordinate state changes for unsynchronized objects like device_machines in multiplayer.
Note that automatic doors which open when players are nearby do not require synchronization workarounds, since player position is part of the netcode.
Bipeds spawn on the client when spawned on the server, and die when killed on the server, though nothing else about them synchronizes. Because biped death is sent over the network, a map script running on the server can spawn a vehicle to crush the biped and trigger an event for clients which detect the death of the biped via their map script.