As I edit, one of the most common errors I have found (though there are many) is when authors do not know whether a word is supposed to be two words, one word, or a hyphenated word. Using Merriam-Webster's Collegiate Dictionary: Eleventh Edition, allow me to share some words and their actual spelling.
all-time (as an adjective)
ice-skate (as a verb)
shutdown (as an adjective)
For the sake of needing to get this blog up, I will post it now. I will add to it as I come across more words during my editing.