Magical Side-Effects

Side-Effects and Expectations

Magic without a framework to work within leads to unbounded side-effects. Science (Physics, Chemistry, Biology, Alchemy, etc.) supplies a framework which, while magic may bend their rules, supply the rules that naturally will restrict the potential of magic.
Without any such framework (such as after the The Burning of the Library of Guines), magic will begin to grow unbounded. As any magician may cause a side-effect with their spells, those side-effects become a part of the unspecified framework the magic occurs within. However, with the framework being unspecified, those side-effects will begin to grow and multiply until eventually magic would become untenable and could potentially destroy the culture or even the world.
Because of this, some form of framework for magic to be bound within, while not necessary for the magic itself, is necessary for the continuation and perpetuation of that magic and the culture that embraces it.
While science may supply a valid framework for magic to exist within, it isn't the only such framework. For example, Lexology and the strict set of rules imposed by spells cast within its framework can also give meaningful boundaries to magic.

Necessity

While some amount of side-effects are always going to occur, they aren't necessary for magic to happen. Were a magician able to perfectly believe the entirety of the universe, there would be no space for side-effects to occur. However, without perfect and complete understanding and belief in the universe (or multiverse), space is always left for unexpected outcomes.
It is believed that these unexpected outcomes will always occur as some form of cost of the magical spells being performed, but whether that is actually true, or if it's only true because it's believed to be true is unknown.

Example

Imagine a magician, Aaron, that wishes to teleport themself from their business to their home. Were they to tell someone, Bill, that they're teleporting home, Bill might expect Aaron needs to have been to his destination before.
Aaron's teleportation side-effect: minimal.
Now if Bill wants to teleport from his home to the beach, knowing Aaron had been both to his home and business, he may think he needs to get to the beach first. So Bill takes a trip there. On the way, he stops in a tavern and over dinner tells someone, Carl, that he's going to the beach so he can teleport there any time he wants.
Bill gets to the beach, and then teleports back home. But when he gets home, he's as tired as if he walked the whole way, so is forced to take a nap.
Bill's teleportation side-effect: exhaustion.
Carl then decides he wants to be able to teleport to the beach too, so he also journeys there. But when he teleports, he has tied teleportation in with the dinner Bill was eating, so when he teleports home, he finds his pantry is empty.
Carl's teleportation side-effect: food loss.
Daryl hears that Bill and Carl had gone to the beach and can teleport there and home, and learns that it makes you tired and hungry, so when he teleports, he finds he doesn't have to go there, but does know that it's going to be draining physically. So he teleports to the beach that Bill and Carl were at, but when he gets there, he's completely emaciated and can't enjoy his day. Nor does he have the energy to travel back home, so he's stuck at the beach while he recovers.
Daryl's teleportation side-effect: physical drain.
And so the effects continue to grow, with Ethan draining not only himself, but others around, Fred killing a flock of geese, George arriving with feathers and wings, Henry flying there quickly, and so-on.
In practice, the effects don't change with such rapidity, as more of the spell is seen or expected, and changes being smaller or rarer, but the example demonstrates the principle of unbounded framework side effects and the dangers they produce for both magicians and society.

Manifestation

Side-effects of spells begin as small effects at first.  However, as these small effects become expected as part of the originating spell, they will continue to produce new larger effects.  Eventually, the side-effects expected for a spell will be larger than the spell itself.

Localization

Expected side-effects cluster within regions, as expectations are formed from other people seeing the unwanted and unintended side-effects of spells.

Type

Metaphysical, Arcane