I don't claim to be god's gift to grammar or the English language, but here are a few of my least favorite things:
Redundant Acronym Expansion
The Merriam-Webster Dictionary defines the word acronym as "a word (as NATO, radar, or snafu) formed from the initial letter or letters of each of the successive parts or major parts of a compound term". If we expand an acronym, we get more words. This is great, as it decreases the amount of information required to communicate a phrase and is a generally efficient form of linguistic compression.
The problem is when people state an acronym in its entirety and then expand the last word, as in "VIN number", "RAM memory", and "ATM machine". ATM stands for Automatic Teller Machine, so the trailing "machine" is redundant—when the phrase is expanded, you get "automatic teller machine machine". In addition to being nonsensical, it decreases the compression's efficiency.
Extraneous Quotation Marks
SALE "TODAY"!
Is it today, or isn't it?
WE SELL CIGARETTES! "LOWEST LEGAL PRICE!"
Are you charging the state minimum price, or aren't you? Did the manager simply say that and you're quoting her/him but really charging more?
Plurality Indicated by Apostrophes
Plurality is not indicated by apostrophes. "Look at the pile of carrot's!" means "look at the pile belonging to carrot", not "look at the pile of carrots".
Self-Contained Double Negatives
In addition to not being a word, "irregardless" means the opposite of what people who use it mean when they say it. "ir" is a negative prefix and "less" is a negative suffix. The correct word, "regardless", means "without regard". Negating it with "ir" makes it "without without regard", or "with regard".
Quote Unquote Stuff
Quotation marks define the beginning and end points of a literal, and the stuff in between them is what the quoted source said. Therefore, the phrase "Mr. T says quote unquote I pity the fool!" is wrong ("jibba-jabba" in T-speak), because nothing has been quoted! "Mr. T says quote I pity the fool! unquote" is correct because it actually quotes Mr. T's statement.
It's "nuclear", not "nucular", damn it!