trigger_once

XashXT

trigger_once

Spawnflags

SF_TRIGGER_CHECKANGLES 8 (borrowed from Quake; the trigger works only if its angles and the angles of the object are pointing in the same general direction on YAW)
SF_TRIGGER_ALLOWPHYSICS 16 (tells the trigger to respond to all physical objects, not necessarily func_physbox)

Option

netname Filter for activating only objects of a specific class (classname) or with a specific name (targetname).