I read a lot of raw copy all of the time. Like most things in life, common mistakes get made. You would be completely surprised what are some of the most common. So for your better edification, here are the top three most common mistakes I see made in manuscripts.
The never-ending quotation marks -- this one seems strange to me. In dialog, writers will put quotations after every sentence in a string of dialog. It goes like this ... "I want to go the grocery store." "I'm going to buy Frito's for lunch." "Then I'm going to come home." Until recently I had never seen this done. And then out the blue several manuscripts started coming in with this dialog written this way. The correct form is: "I want to go the grocery store," she said. "I'm going to buy Frito's for lunch. Then I'm going to come home." Do not put quotations marks after each sentence. Put quotation marks around the entire string of sentences said by the same character all at the same time.
Now I "lay" me -- is it laid, lay, lie, lying. Here is my advice, Grammar Girl gives you a handy reference chart for lay. I suggest you go look at it. I still get confused and struggle with this one. So, I have taken to referring to the chart whenever my mind is in grammar overload.
Passive voice is the bane of my red pen -- new writers use passive voice as routinely as they eat. The problem with passive voice is it clutters your writing. Passive voice clutters your writing. (You see what I just subtly did?) Passive verbs include is, are, was, would, etc. Again, hope on over to the witty Grammar Girl where she will give you a quick lesson in passive voice and know some "is" out of you.