It has to do something that was completely unexpected, but still in the same vein as the spells original purpose.
Good Example:
W
"Stand behind Braum" "Accidentally body slams his ally"
Oops...
Bad Example:
Q
Draven's mustache catches on fire.
auto-attack
Gets its tentacles stuck in the (female) enemy champion.
Rammus crossed the line when he was attempting to taunt the enemy and the enemy's self esteem goes so low the champion commits suicide
Presses Q
Bomb explodes in Ziggs face...
Activates Q
Fails to catch axe.... I wonder what the announcer would say, something like An ally has committed suicide or An enemy has commiited suicide
Uses Q
Twisted fate gets a paper cut xD
Right Click
Suit breaks down. Stuck in middle of the lane not able to move and die to enemy laner. Team flames and all chat to report me for feeding and going rage afk in fountain when I respawn. Account perma-banned...
I just queued up with Veigar to see how bad it is...
Stun takes so long you think it will never go off, then it stuns you.
Acceleration gate
Shocks allies and super powers enemy projectiles.
R
The bazooka was facing the wrong way so she ults herself
* Presses R*
Somehow ults a minion aside of the enemy champion.
tries to finish someone under their turret with W, of course after landing Q.
She overshoots and face-slams into the turret.
Of course, mixes up his turrets with cotton candy dispensers, missiles blow up in his face, and the grenade is still useless.
R
Ults 
, couldn't see clearly through his own darkness and ults his shadow.
- tosses boulder
hits teammate in back of head, knocking them out for rest of the match
Sand soldiers rebel
casts his Q too close and his copter explodes
Spiderlings eat her
Steps on her own Hate spikes
URF mode returns and he flies away
Oh jeez where do I start...
jumps and lands on top of an ally and roots them into the ground
Throws Orb
Attention is transferred to a strange moaning sound in the bushes
Forgets to catch Orb flying back at her, Orb whacks her head off...
Tumbles Face first into a wall
Looks into his gem shield
Sees reflection of himself
Gets stunned by outrageosness
Accidentally picked up too high by howling gale and dies of ionization in space
knocks up his own team.
Impales himself
Starts choking (R)
Hits himself (and all allies) with ult
Realizes it takes millions of years to evolve (R)
Uses ult and vanishes from the match
meeps go on strike
Need I say more?
Tries to use his W, but it fails because someone sprayed "no more tears" shampoo in his eyes.
(yes I know it doesn't work like that and the stuff burns like hell, but he's a freakin mummy)
he charges in, and trips during his ult
-
Q - She hurst her wrist when she strikes a tower and hops around for a moment holding said wrist.
W - Leaves her exsausted.
E - Only a tiny little ember is created and it lazily floats to the ground doing nothing.
R - https://www.youtube.com/watch?feature=player_detailpage&v=ycyPNO3ll-k#t=107
Rite of the Arcane (R) after being heavily fed actually destroies the map leaving him alone with just Nasus and Renekton alive watching t him "not again please.."
Idol of Durand (R) Stone gets too much thick and can't get out
Righteous Gust (E) Flaps too hard.. allies flying down the Howling Abyss
Q (Bouncing Blades): Katarina is unable to control where her blade bounces. It comes back and hits her in the face.
W (Sinister Steel): Katarina's daggers accidentally slip from her hands during the attack, leaving her unable to fight until she retrieves them.
E (Shunpo): Katarina stumbles on her target when she teleports to them, leaving her stunned for a second.
R (Death Lotus): Katarina gets dizzy and nauseous after spinning, keeling over and puking in front of her at the end of it.
tried firing his rockets. . . uh oh. . . technical difficulties. . . looks like he launched himself. . .
ult
accidentally knocks up his whole team, causing enemy yasuo to ult
ult
sword blows up in face
I'll finish off , I suppose.
Q: Braum punches his shield a bit too hard, damaging himself and giving himself the "Broken Fingers" debuff, causing him to temporarily be unable to use his Q.
E: Braum accidentally puts his shield up backwards, causing all enemy projectiles to pass through and all allied projectiles to be blocked.
R: Braum slams his shield into the ground... and it gets stuck, temporarily immobilizing him while he pulls it out of the ground.
As for last played champions:
: R: The force from Sona's ultimate causes the Etwahl to snap in half, rendering her useless until she backs and buys another one.
: Q: Sejuani forgets to hold on tight enough when she Qs, and the resulting force launches her off of Bristle.
R: Sejuani uses her ultimate and accidentally throws her flail instead of her bola, rendering her unable to use her W (Bristle could still bite people for autoattacks) until she picks it back up.
W
Loses his voice
So...
spits up a hairball instead of poison? No, I got it. Chokes to death on her own cloud of poisonous gas. Or she ults and it hits a mirror, so she turns herself to stone.
I tried to polymorph the enemy adc, but ended up giving him a speedboost instead.
"Enemy double kill!"
My adc: OMG report Lulu for assisting enemy team.
