"I'm not sure how I found this site, but I've read through some of the articles on the don'ts and they make sense, for the most part. Sometimes, it's better to hear the don't do this rather than the do this.
I think one common theme is spelling, grammar and punctuation. It shows someone who doesn't know their own language and doesn't care."
http://www.screenwritingspark.com/the-ultimate-guide-to-screenwriting-mistakes/