Specialty contact lenses can pump up a look or add the finishing touch to a Halloween costume, but they can also introduce dangerous bacteria to your eyes or cause other damage. If the lenses are too big, they can rub the surface of your eye, scraping the cornea, and if they are too tight, they can prevent the cornea from getting the moisture it needs from your tears, which can cause serious irritation and corneal infection. On the extreme end, wearing the wrong size contacts can cause corneal ulcers, which, aside from being extremely painful, result in light sensitivity and vision changes if untreated.
If you do want to try out specialty lenses, make sure you get them from your doctor, who can provide you with a quality brand that is in the right size for your eye. Be wary of online sellers, costume shops, and salons selling these lenses; according to the FDA, anyone who sells lenses without seeing your prescription is breaking the law.