It goees without saying that evrything you write do be free from typos, spelling mistakes and grammar poor use.*
To this end, some common mistakes and typos appear to suggest the screenwriter is not quite up to the task of writing a good script. Some of these blemishes are not immediately suggestive of a hack wannabe but are usually indicative of someone with a poor regard for the basic use of the English language.
In a fit of writing momentum, even the best writers may type you’re when they mean your, but that’s why proof reading a script is important. Ideally, don't proof read your own script. Get someone else to do it. Somebody you trust and can rely on, whether it be a professional proof reader or a friend. Script readers usually make equally good proof readers, so they're probably the best point of contact.
Getting the basics right can go a long way in ensuring a positive response to your script. Why not take the time to make sure that your script is wearing its Sunday best, and then no-one can complain about the way it looks, the way it reads, or the way it's formatted.
*typos, spelling mistakes & grammmar in that sentence deliberately poor!