To get the vanishing plasma gun on level 8, Final Outpost:

(Note: For a YouTube video covering this topic click here.)

When you first walk through the red-keyed door and Activate
the nearby switch, a plasma rifle just behind the switch
disappears.

After activating that switch, turn around and look for a
flickering viewscreen on the wall.  Step up to this screen
and Activate it like a switch.

Whenever you Activate the viewscreen, a picture of the
plasma rifle's location is shown.  The rifle rests inside a
4-walled square area which is open to the night sky.  In
each corner of the square is a camera that looks diagonally
across the square.  Each time the viewscreen is Activated,
one of these cameras is chosen at random to provide the
picture.  (Because of this, it may seem that the weapon is
jumping from location to location; but in reality, only the
camera angles are changing.)

Turn right, away from the viewscreen, and go into the room
nearby where 8 tall pillars stretch from floor to ceiling.
The ceiling of this room is also the floor of the square
area where the plasma rifle is now located.  All 8 of these
pillars extend upwards to pierce the ceiling, and the
plasma rifle is resting on top of one of them.  (The plasma
rifle's location is chosen at random each time this level
is played.)

Each pillar has 4 switches on it.  Choose any ONE pillar
(it doesn't matter which one), and Activate any one of its
switches.  The pillar will drop down to floor level.  If
the plasma rifle is on this pillar, simply grab it, and
you're done.

If the plasma rifle was not on that pillar, do not activate
another pillar yet.  (ONLY 2 PILLARS CAN BE ACTIVATED; Once
the second is activated, the rest become unresponsive.)  Go
back to the viewscreen and Activate it again.  In addition
to the plasma rifle, you'll now see a hole where the top of
the lowered pillar used to be.

Activate the viewscreen as often as necessary to see where
the plasma rifle is located in relation to the new hole in
the floor.  In this way you can determine which remaining
pillar supports the plasma rifle.  Activate that pillar to
acquire the weapon.