Anyway, during the Rise of the Runelords campaign I'm hosting, the players met an almost dead pit fiend entrapped in a certain dam. He offered a bargain in exchange for his freedom, exactly a Wish. The party, being composed mainly by good guys, refused...but one of the members later sneaked back into the pit fiend chamber and asked for TWO wishes if he had to release the devil. They haggled a bit and the pact was sealed: the devil was freed after hearing the character's wishes.
-I want to find again my lost sister, with her bein in full health, no menomations, no mental illnesses and no debts.
-I want my strenght to match a giant's one to be first among men, without having the to look like a giant, being a giant or being a stupid brute.
-He found his sister. But she had been kidnapped by a gang of bandits and the party had to rescue her risking their lives. Also, as soon as she was rescued, the devil appeared, disguised as a noble gentleman, and whisked her away. Afterall, he didn't ask for the sister to stay...
-He got a +1 to strenght. But now he breaks everything he touches because he's too strong: plates, utensils, beer horns, even weapons (he damages his weapons when he activates critical hits). He's been banned to enter the local brothel because the girls felt pain every time.
To sum up: grant them whatever they want. Give them their deepest wishes...then twist them around in the worst possible way.
You diabolical bastard ... nice job!