Another big mistake of new authors is getting scene tense correct. IE, has vs have. Oddly enough, even I struggle with that somewhat. I think part of that comes from the way we speak. In literature we're required to exact perfect grammar. In speaking, not so much. I've gotten away with using things like "me and you" vs "you and I" so often it hurts. Or using mishmash hack words. For example, around here it's very, very common to use "fer" instead of "for", "gonna" instead of "going to" (I even use this word a bit in my writing because it just flows better in dialogue), "should've" instead of "should have" and so on.
On the flipside you can also get so cautious about using wrong words that you go overboard and start mentally eliminating conjunctions you should be using, such as writing "he had" instead of "he'd", or "we are" instead of "we're". I'm guilty more of this than the first, but I still do both periodically.