A couple of years ago on Classic, when using projectiles with 0 gravity, and a speed of 1, which makes them travel over walls and only collide with NPCs and players, the projectiles weren't destroying when going out of bounds (as in outside of the level or map coordinates), this was causing excessive amounts of server cpu useage, and was subsequently fixed so the projectiles were destroyed.
However while the projectile objects are destroyed serverside, they seem to remain Clientside until you leave the level, it's usually not a problem, but if you do stay within the same level where projectiles are shot from this can lag your client quite alot.
Something else i've noticed is that while you're on a gmap, and there are projectiles that have gravity in mid flight at the point of you entering another level on the map, they destroy clientside and onActionProjectile2 is called, this is somewhat of a problem if you have projectiles that do stuff like drop an explosion clientside upon landing
.