Your mirror images are all disturbing alternate versions of yourself, and thus cause psychic damage to you. 

I cast Prestidigitation to clean my clothes. 

You accidentally use the wrong effect and set fire to yourself.

I cast Fireball centered on myself, and I have vulnerability to fire damage and one hit point remaining. 

The DM remembers you're an evocation wizard with Sculpt Spells so your presumed goal of annihilating yourself goes awry.

I cast Ray of Frost to chill some melted ice cream.

The ice cream chills to absolute zero and is inedible until 10 hours in the sun later

I cast Vicious Mockery

It literally backfires and informs you that your mother was a hamster and that your father smelt of elderberries.

I cast mending

Since you failed to specify what you were mending, that which determines what you are mending if it is unspecified decides that you are mending the universe. The universe implodes on itself, because there can't be a broken universe.

I cast counterspell. I'm not trying to counterspell anything in particular. I'm just going through the casting of the spell in general.

As there is nothing to counterspell, the spell automatically fails.

I cast Hot swooshing sword

It misses and hits your party’s level one sorcerer.

I cast Witch Bolt

Instructions unclear. Which Bolt are you talking about?

I cast Phantom Steed.

You fall through the steed trying to mount it.

I cast Heat Metal

You misword it and accidentally summon a Meat Hotel.

I cast Frostbite

On yourself.

I cast Booming Blade

You misword it as Blooming Blade. All around you and on your sword, beautiful flowers grow.

I cast Steel Wind Strike.

The wind turns into steel. Good luck breathing.

I cast Power Word Heal

You accidentally cast power word meal, and get a nice hearty meal that unfortunately is not hearty enough to heal you, and you don’t even get a short rest because you ate way to much and are just groaning in pain now

I wish for a ring of three wishes with full charges (3)

You indeed receive the ring. But you can no longer give it away, you always safeguard it as your most precious posession. And you slowly are corrupted by its power. You lose more and more of your memory and personality and your skin grows more and more wrinkly until you end up as a cave-dwelling something that bears almost no resemblance to your former human self. The only thing you still remember is that this one ring is your precioussss....

And both nine black horsemen and an adventuring party of an elf, a dwarf, two humans, four hobbits and an aasimar are hunting down the ring...

I cast holy weapon.

The weapon decides you are not worthy and you get smited.

I cast Danse Macabre

The zombies don't listen to you and eat your brains. You die.

I summon Acererak.

He kills you.

I cast Dancing Lights!

The lights dance around you and then you go blind from how bright the lights are. By being blind you dont see the Tarrasque destroying the city and you get flattened. https://www.dndbeyond.com/forums/off-topic/forum-games/87403-kill-the-player-above-you?page=115#c2387

I summon The Great Old One.