Affect and effect.

They're different words, with different uses, but a lot* of the time I see them incorrectly used, and it irritates me.

Affect:
"To have an influence on or effect a change in: Inflation affects the buying power of the dollar."

Effect:
"Something brought about by a cause or agent; a result."

Oh, and let's not forget Mr. Apostrophe, which does not pluralize (pluralise, if you will ). "The cat's" does not mean several cats, it refers to something that belongs to the cat.

*Also, it is "a lot", if you are referring to a large number of things, "allot" if you are portioning something out, and "alot" never.

Yeah, I dislike bad grammar.