I feel I should mention that I wouldn't pass on a script SOLELY because of grammar errors...but they're a pet peeve and they can take your readers out of your story.

A screenwriter writes for two audiences, the Reader and the theater Audience. That's an important reality to grasp and execute. Always remember and never forget... the audience won't see your (not you're!) genius if, as Amanda says, you "take your readers out of your story."