It bugs me when:

• "a lot" is spelled as one word
• people use "less" when they should have used "fewer"
• "however" is dangling in the middle of a sentence when it should have been preceded by a semi-colon
• "cannot" is spelled as two words
• bulleted lists use semi-colons at the end of each bulleted item (although this is not wrong, I see it as pointless since the bullets imply a semi-colon)
• people use the word "utilize" to make themselves sound smarter
• words that shouldn't be capitalized are capitalized