When a projectile is Area of Effect and repeating, like the Stinking Cloud or Web, putting Cast Spell as the effect for that projectile makes the one who launches the projectile cast the spell in the Resource field on those who enter the area, from one to the next in turn, again and again. When they leave, the caster stops; when they step back in, he wakes to that fact and resumes.
This can have interesting uses, I think. Imagine a spell that creates a large trap; so long as enemies are inside, the caster throws magic missiles at them as free spells. The effect can also be Use EFF File, summoning minions and ordering NoAction() on creatures or forcing them to attack each other or flee the area. It's also possible to station a minion permanently to maintain this kind of zone around itself, and let the zone's effect be #186 - Move To Area. It would be a constantly active teleportation gate that could be conjured anywhere without the need for area editing. It could be switched on and off, if necessary, with a global or shout passed to the minion.
And then, weapons can also send A-o-E projectiles. They still require a hit, but the Area trap size/explosion size can be 0, yet repeating and causing the attacker to throw one spell after the other on the victim - like a series of Minor Drains or something custom-made. I've tested on Arrows of Fire, simply making their damage repeating, but this could be an instant Cast-spell instead. There are all sorts of AD&D monsters with tentacles that latch on and suck blood or brains (mind flayers, after melee hit die in 1d4 rounds, IncrementGlobal) or teeth that gnaw or burning whips.
