You’ve just become invisible, including your clothes. Hmm ? Oh no, you can’t go back. Watch out for that bus !
I wish for a one shot, permanent, guaranteed cure for all types of cancer.
Oh, you get your Universal Cancer Cure ™, but at the cost of everyone on the planet becoming cyclopses (one big eye) to ensure immunity.
These things have to balance, ya know-an eye for a tumor.
I wish I had a pet cyclops who would smoosh the people I don’t like (starting with right-wing pundits like Limbaugh and Hannity) with his huge war club.
Granted. Oh, you also discover a secret about yourself that fills you with self-loathing. Your cyclops senses this dislike and you spend the rest of your days running and hiding from a huge war club and the cyclops attached to it.
I wish I had three wishes…
Granted. Too bad you didn’t know before you said:
“I wish I had a beer right now.”
“I wish you’d shut up!”
and
“I wish my neighbor would stop making so much damn noise!”
And all of three got used up.
I wish my dog Tucker was still alive and healthy.
Granted. You’re not.
I wish that my apartment was clean.
Granted. A team of severely OCD catburglars nick everything you own, leaving your appartment squeaky clean.
I wish the Earth would be consumed in a rain of nuclear fire, muahahahah !
Granted. Unfortunately, it’s your subconscious that’s calling the shots and it wants gold all the time. Try not to touch yourself and hire someone to feed you. Don’t worry. you can afford it.
I wish I had a nice motor boat.
Certainly. BANG. All types of cancer you may have been suffering have now ceased all metabolic functions. As, unfortunately, has the rest of you. 
/hijack
Bzzt! By granting wishes which have already been granted (thus ignoring Kobal2’s most recent wish), Uosdwis R. Dewoh’s & Malacandra’s mouths are erased for 24 hours (a la Keanu Reeves in the Matrix).
Granted. After your Victory Dance (having disposed of Ronnie Raygun, Infidel Castro, and Khadaffy), you wander the desolate ruins, eventually losing your mind in the process. The rest of us were saved by the Improbability Drive and are perfectly fine and happy on Earth 2.
As you wish; but you survived, and the spirits of every human being on the planet tortures you so. Eventually you commit suicide (having survived for a time on the moon in a spacesuit), only to discover that you have now been promoted to the Really Really Evil Dude Ruling All Previously Created Planes of Oblivion™. But with no one to torment any longer, you live an eternity sitting on your throne, unable to move from that spot or enjoy anything. Also, your mom shows up and tells you that she is actually your father. Confusion ensues.
I wish, I wish, with all my might, to know the outcome of every probability that personally effect me before it does.
Bah, I made a typo that I can no longer correct in my above post. Curses, aluminum foil again!
Okay, but knowing every single outcome for every single probability for everything that could ever affect you drives you mad! MAAAAAAD!
I wish I could take any kind of drug from ibuprofen to LSD to caffeine and reap all of the positive effects and none of the negative ones.
That’s fine. Now all of the major pharmaceutical companies, fascinated by your bizarre physiology, pester you endlessly to be a test dummy for them. After you freak out and kill a particularly annoying guy from Pfizer, you’re locked away in prison for life. But at least the infirmary drugs don’t mess you up!
I wish to provide some momentous service to humanity before I die, and to be remembered fondly and with honor by everyone afterwards.
You assassinate the person who everyone believed was going to be the next Hitler. Statues are built in your honour. Songs and movies are made about your life.
It’s a shame the guy you killed was actually the Second Coming of Jesus. Earth becomes a living Hell, and you are condemned the the actual one.
I wish I could find my soulmate.
You find your soulmate – crushed under the two tons of twisted metal that is your car. You are sentenced to vehicular manslaughter wherein you find your cellmate (who also claims to be your soulmate, but we already know that is impossible).
I wish to find an authentic pirate treasure, ripe for the pickin’…
Granted. You found the Somalis’ treasure trove ! JACKPOT ! Millions and millions of dollars, in used bills. Used, marked bills. You’re flagged by the IRS and Treasury on your first purchase, are convicted with money laundering, and spend the next 25 years in jail.
I wish I was awarded a Nobel Prize.
You’re awarded the Nobel Prize for Peace, after bringing the Israelis and the Arabs to the table and hammering out a peace agreement. A week later, alas, war breaks out, and the Swedes huffily demand the prize back again.
I wish I didn’t feel compelled to make wishes.
Wish granted! You no longer feel compelled to make wishes. However, you do feel compelled to make others make wishes. The others are getting pretty annoyed with it, especially since their wishes are backfiring as well.
I wish truth, harmony, and justice were tangible items…
NO problem! Truth, harmony and justice are all tangible. They weigh an awful lot. And they’re sitting right on top of the rubble heap that used to be your house.
I wish I were a member of my favorite band.
Done. You’re now your favorite band’s drummer. No one has ever heard of you. Don’t worry, you’ll probably still drown in your own vomit someday, so it’s all good.
I wish Firefly hadn’t been cancelled