Backfire the Spell

You turn everyone into giant tigers and they maul you.

I cast Minor Illusion

You make minors look like adults. Chaos ensues.

I cast Jim's Magic Missile

You don't have any gp and are arrested for tax evasion

I cast dimension door

You are blinded before you can cast the spell.

I cast True Strike. Good luck messing up a spell that's already messed up!

You gain insight into your enemy's bowel habits.

I cast Grease

Your opponent becomes covered in grease, making them harder to hit.

I cast create bonfire

that should be easy to screw up.

You instead create a bonfire cast, a mold that when filled with molten metal creates a figurine of a bonfire.

I cast  command

You command yourself to walk off a cliff

I cast Ray of Enfeeblement

You hit yourself, roll a natural -20 on your save, and explode into a cloud of venomous beetles, which then all roll half damage on attacks to kill each other, resulting in a cloud of ever-warring beetles.

I cast Summon greater demon

You summon Asmodeus.

I cast Feeblemind. Please don't just make this target me; that's overdone!

You shatter the mind of a god, and they are so stupid now that the earth itself shatters.

I cast Sleep on an enemy scout.

The scout gets ready for bed, and won't get to sleep until you come to his bed and sing him a lullaby.

I cast Motivational Speech.

You motivate your companions to kill each other for sport.

I cast magic stone.

Stones raise into the air and come raining down on you, you take 1d6 bludgeoning damage per stone, and there are at least 100, so good luck surviving.

I cast Ice Knife

The planar ally kills the kitten in the most brutal way ever.

I wish fir a completely harmless kitten that id=s real flesh and blood and fur and organs and blood vessels etc, etc

You get a kitten drenched in blood.

I cast modify memory to get that thought out of my head

You create a race of cat modrons that conquer the multiverse (and also turn on their creator).

I cast mass suggestion.

You suggest that they make you a pot of soup, so they boil you alive in a kettle of stock and vegetables. 

I cast Find Familiar