Performing magic has some unwanted or unnecessary extra consequences.
In some fictional worlds, using magic doesn't only get a certain desired effect, but causes something else to happen as well. This could be something small, like snowflakes appearing every time a character shouts "Shazam!", to more serious issues, like the appearance of items that could be potentially life-threatening. Depending on the story, this might be a big problem for if the magic practitioner can't handle the side effects very well, or it might just be a cute little quirk, likely of a symbolical nature. In any case, the side effects do not have to have any influence on the magic spells themselves, or threaten the practitioner directly. Compare Equivalent Exchange, where the use of magic does have a direct influence on the practitioner. See also Electromagnetic Ghosts, which deals with one specific side-effect.