In the Desert Multiplayer map, there is an area where you can take cover and see 
through the wall. In order to do this, you must have Bases 2 and 4 open. Go into 
the cave temple base, then go through the hall, and up the sand hill, not the 
stairs. Go in the little courtyard from either entrance then turn left. There 
should be a bowl near the wall. Stand as close to the wall and as far in as you 
can into the wedge created. While standing, go into cover. If done correctly, 
you should be able to see through the wall. This is most effective during the 
Infection game type with the Plasma Rifle. Hide there and whenever you see a 
skeleton, use the secondary function (invisibility). They will usually walk out 
because it is very difficult to see you.