Be careful what you wish for...

[The Elf-warrior]

I wish Shelob had just had a really big pig out on Orcs and so couldn't chase Frodo and Sam.

Your wish is granted. But Frodo and Sam are spotted from the tower of Cirith Ungol. Frodo puts on the Ring and disappears but Sam is captured. Later Frodo is captured by one of the Nazgul and thus Sauron regains the Ring.

I wish that all of the sons of Feanor had been killed trying to take Dior's Silmaril.