10. Giving Names To Very Minor Characters
There's a limit to how many names you want flying around the pages of your script, and a common newbie mistake is to give absolutely EVERYONE a first name, even when they're only there to perform a single task. So characters like "Policeman 1" doesn't need to have a first name or a second name - "Policeman 1" is just fine. Same goes for any taxi drivers, shopkeepers, and random homeless people who don't hold any story weight.
9. You've Used Unnecessary Parentheticals
It's pointless to offer line directions in parentheticals, because - ultimately - it won't be up to you, and an actor will say the line as it needs to be said, depending on the mood of the scene. You don't need to mention that any lines should be said "darkly" or "sarcastically" - the line itself should ideally reveal how it should be spoken, and additional direction just takes up room (and suggests the writer is an amateur).