I’ve been given the chance to place a curse on an enemy, whoever you like. However, it cannot be directly or indirectly fatal (if they kill themselves because if it, that’s not your fault, but no curses about massive organ failure).
My favorite idea: everytime they sneeze-spiders come out of their nose.
I curse Love Rhombus to lose his knack for conversational geometry.
I curse a particular store clerk to get an inappropriate boner whenever he’s speaks or is spoken to by anyone in a position of authority (boss, cop, customs agent, bouncer, landlord…)
I curse my boss to have to experience the hurt she doles out every time she says or writes something snippy, unkind, or condescending.
I curse my boss to get wrongfully convicted of a felony, go to a maximum security prison, and receive the sort of treatment given in prison to cute little pretty boys who were born into wealth and privilege.
I curse the executives at Time Warner Cable…they will being to hiccup…and they will never stop!
Bwaahahahaha!!!
Get early parole?
A friend suggested a really nasty one: I curse someone to have their internal plumbing rearranged so that they must excrete through their mouth. Ugh.
Somewhat harsh by the standards of the OP but it’s too juicy not to mention.
Servilia’s curse on Caesar from HBO’s Rome.
“Gods of the Junii, with this offering I ask you to summon Tyche, Megaera, and Nemesis so that they may witness this curse. By the spirits of my ancestors I curse Gaius Julius Caesar. Let his penis wither. Let his bones crack. Let him see his legions drown in their own blood. Gods of the Inferno, I offer to you his limbs, his mouth, his breath, his speech, his hands, his liver, his heart, his stomach. Gods of the Inferno, let me see him suffer deeply, and I will rejoice and sacrifice to you.”
My favorite curse is that stupidity should HURT. However, I’m afraid that in some cases this would be directly fatal…
Actually, it looks like the OP more wants people to come up with fun/creative/annoying curses, not imagining your least favorite person suffer. 
Oh, you can imagine it. Just make it interesting for the rest of us to imagine, too. 
May the hairs in your arse grow at an alarming rate.
I curse my former boss so that whenever he starts saying something pompous, arrogant, pretentious, insulting, condescending, inappropriate or manipulative, he shrinks down to a tiny size like a little toy soldier. It can only be undone with some self-awareness and genuine apology.
When he shrinks, people are allowed to put him in scary, but not immediately life-threatening, situations. For example, in the toilet bowl with a turd, or in the fridge, or in the bin. I mean, he has to be rescued, but a lot of fun could still be had.
I curse my boss with explosive diarrhea whenever he says something stupid
I would love to place a curse on the entire world: Everyone gets what he/she TRULY deserves
I wish to curse those who are ignorant by choice, those who do not want something so rude as the facts to get in the way of their bias, preconcieved notions, prejudice, fear and self righteous bigotry.
I curse them to live in a universe where their ideas ARE correct, true and factual.
I curse Judy at work to have her false teeth fall out when she’s talking to a group of people.
The best and easiest curse for someone in Management is that the people above and below them no longer believe their lies.
I curse all Board of Directors and Officers of Corporations to suffer uncontrollable medically unstoppable diarrhea until they pay all the taxes they should pay and NOT just just the taxes they can get away with paying.
I cures everyone who is dismissive towards migraine sufferers to being afflicted with the first of many, many migraines.