3. Poking yourself in the eye with a sword (or other prop)
Probability: Depends on how clumsy you are
Shockingly, there's no data for just how many people actually need their pirate eye patches on Halloween after poking themselves in the peepers with cheap props, but witches brew + a toy sword isn't always the best combination. If you're at all clumsy, and decide to dress as any sort of character that wields a sword, machete, knife, or shish kabob, well, enjoy both of your eyes while you still have them.
2. Tripping over your too-big costume
Probability: Pretty likely, if you're someone who enjoys adult beverages on Halloween
For some reason, many Halloween costumes tend to involve long, draped outfits -- if you're not going for the sexy look -- which makes tripping almost a guarantee, especially considering the general debauchery you'll be getting into.