Sharpened Hooks: Accidental Occult Science

Consider the following:

Magic is a collection of symbols assembled to focus the will.  The symbols frequently aren't important; the important part is what they mean to the user.  This is a very postmodern view of magic, but most games relate to this in some way.  D&D is one of the last holdouts: rituals require specific components to work, although in 4e, those components are ill-defined such that there might be a wide variety of reagents to be used for rituals.  It can also be argued that magic is taught by mages, and they probably have a specific method of doing things; it's possible that the Vancian magic of D&D is completely cultural, and that another fantasy culture might use something different.

The most important fact from this is that magic is mysterious and unreliable (D&D is a borderline case as magic frequently just works, though some sources imply that magic is not as reliable for most NPCs).

Now consider Call of Cthulhu.

Magic in Call of Cthulhu is just different.  Yes, it's mysterious and unreliable, but not because it draws on symbolism and human will.  It's mysterious and unreliable because you don't understand it.  The magic of the Cthulhu Mythos isn't magic as we understand it; it is composed of scientific principles we have yet to discover (and Lovecraft being Lovecraft, we can't discover those principles without destroying ourselves and radically altering our civilization to something insane and lawless by the standards of our current society).  You don't inscribe a weird sigil while intoning a weird chant and holding a weird image in your mind because this focuses your will, you follow these steps because magic is a recipe or a chemical reaction, and if you follow the steps exactly, magic happens because you hit upon the right combination of factors (the Elder Sign is the classic example: it's not a cross to be brandished but an extradimensional hyperbarrier that physically blocks entities with extradimensional anatomies).  If you mix vinegar and baking soda, it makes a big mess whether you understand redox reactions or not, and any would-be anarchist can make a Molotov cocktail whether he understands the full interactions of the combustion reaction or not.  Likewise, a lot of historians have made the observation that the person who invented gunpowder and the person who developed gunpowder were probably different, because the Chinese alchemist who was looking for the Taoist elixir of life probably detonated his lab when he accidentally discovered gunpowder.  Then, some other guy wanted to make a big boom, too, and replicated his work very carefully.

Keeping this in mind, it is possible that a human would stumble across the principles of the Yog-Sothothery without fully understanding them.  Note that this is somewhat different from, say, the sorcerer's apprentice; the sorcerer's apprentice should know better, and the trope usually implies some moral about responsibility.  On the other hand, the accidental scientist never had a chance.  He didn't know he was meddling with obscene power, but now he's stuck with the consequences of his actions.

Obviously, these are the sorts of things from which plots originate.

Imagine the PCs find strange events, and find the evidence of somebody summoning something that cannot be put down or some similarly strange and/or horrible event (or maybe the event is somehow beneficial, but it is unnatural enough that it needs to be stopped anyway).  But rather than the suspected cultist activity, they find someone who might be schizophrenic, or frightened, or dead.  It was an accident, and an accident that could possibly be replicated by anyone who has a similar idea.

Note that this can be used in scenarios other than horror: even though magic is primarily symbolic, maybe someone hits upon the right symbolism to focus his or her will.  Accidental curses are a common element of fantasy, after all.