One thing I myself am often guilty of is making magic always "active". Magic is something you do to warp the laws of physics to be different from our own reality, it isn't really part and parcel.
To a degree I long ago started working in "medieval science" , disease really is caused by vapours for instance, there are only 4 elements and heavier things fall faster. But with magic I find myself often unwilling to put superstitious magic into the fabric of the world.
Let me give you an example. When undead is created its because someone used magic actively, either with a spell or a magical ability to cause the rising of more undead (like a vampires bite). I have a hard time say, having undead spring forth if a player kills someone on a full moon without leaving 2 copper for the soul to pay their fare on the afterlife. Or if the PC's kill a traitor but don't bury him on the crossroads, or if you light a candle in the vicinity of the recently deceased.
Likewise if a player kills a spider I don't make it rain, if they break a mirror I don't make them more likely to fumble for 7 years.
But maybe I should, what does everyone else think?