Those who hunt by night will tell you that the wildest, most vicious of all animals is the tiny shrew.
The shrew feeds only by the dark of night. He must eat his own body weight every few hours or stave to death. The ravenous shrew eats everything, bones, flesh, marrow… everything.
Intro from Attack of the Killer Shrews 1959
Magic should have side effects, both expected and unexpected - fortuitous and deplorable. Expected side effects add anticipation, while unexpected ones increase the drama of the scene. The result can be comedic or dire, it dpends on which circumstances work best for your game.