The Myth Projectile Handling Problem
(this problem occurs in maps which use a single projectile that is
constantly being picked up and dropped by monsters)

By DeadMan of Project Magma

This is concerning the problem and solution for handing projectiles or
artifacts in the TFL or M2 engine for use in new gametypes.

The problem-
Timing problems in the Myth engine allow any projectile (including
artifacts) that is nudged while being picked up by a monster to be
"split".  In effect this creates two instances of the same projectile,
one in a monsters possession and one on the mesh.  This can cause
crashing when the projectile falls back onto the mesh.

Possible crash scenarios-
1: A monster picks up a projectile as it is nudged, or scripting gives a
projectile to a monster as the projectile is nudged.  The same
projectile is now both on the mesh and in a monsters possesion.

2: After Scenario #1, another monster picks up the projectile left on
the mesh.  Two monsters now carry the same projectile.

When crashing occurs-
Scenario #1: if the projectile is not flagged as "becomes dormant at
rest", when the monster loses posession of the projectile it will cause
strange behavior or a crash.  If it does not cause a crash, films will
not play properly and testing revealed that instigating the problem
again caused a crash every time (tfl engine).

Scenario #2: if any of the monsters lose possesion of the projectile in
any order after two monsters have the same projectile, crashing occurs
100% of the time.  Take in mind strange instances of this problem can
occur just after Scenario #1 occurs if map actions give the mesh
projectile to another unit (this is the same problem, 2 monsters with 1
projectile, but it can be hard to track down).  Also take in mind that
in rare occurances the mesh projectile from Scenario #1 can be doubled
as well, in effect creating both the scenarios (3 projectiles, 2 in
monster possesion).

Side effects-
A sure way to tell that Scenario #1 has occured is that a monster has a
"-1" projectile or artifact in it's possesion on the bar at the top of
the screen.  This is not due to an error with the artifact, but actually
an error in the Myth engine.

Solutions-
In fear and tagedit: Make sure the projectile is flagged as "becomes
dormant at rest" inside the projectile flags window.  This will prevent
crashing on all instances of Scenario #1 when the monster loses
possession of the projectile;  the projectiles on the mesh will actually
dissapear, or "recombine" with the projectile thrown or dropped from the
monster.  This can happen over and over with no crashing problems. 
However, the myth engine will still crash if a monster loses possesion
of a projectile after Scenario #2.

Scripting: Although you could get creative with this, the only way
scripting will work the present way I have it organized is if there is
only 1 projectile of a certain type allowed within a testable area of a
map and unit possesion.  The strategy with scripting is prevention of
the problem.  Also take in mind that unless the projectile is flagged as
"becomes dormant at rest", GEOMs will not be able to detect the ball
reliably when either Scenario #1 or 2 occurs.

The strategy for Scenario #1 is to prevent it from turning into Scenario
#2.  To do this you first need actions that can detect 2 projectiles
(when there should only be 1).  For this scenario, a great way is just
to check that a projectile is both on the mesh and in a monsters
possesion.  Once this condition is met, you will need to remove one of
the projectiles, either from the mesh or from the monsters possesion (in
tfl, you cannot remove a projectile from a monsters possession however).
 Either will work, and if you choose to get rid of the mesh projectile,
the "-1" in the title bar for the projectile-carrying monster goes away.

The strategy for Scenario #2 is to prevent a crash.  A good way to do
this is always have a GEOM action that will look outside of a
projectile-carrying monster for monsters carrying the same projectile. 
If this condition is met, one of the projectiles must be removed to
prevent a crash.  In TFL, because you cannot remove projectles from a
monster's possesion, you must delete a monster, however in Myth2 you can
simply remove one of the projectiles.

Always test your actions... It's difficult, but instigating this problem
can be done with some fancy fetchwork and some invincible ghols.  You
can even test on a cleaver or other persistant projectile.