From Grammar Girl:
You could perhaps say, “I ate all the cookies. Big mistake.” Here, “big mistake” is a sentence fragment, but it’s OK because it allows you to emphasize your point. “Which” clauses that appear at the beginning of a sentence or paragraph are likewise incomplete sentences, and you are allowed to use them occasionally.
Can a sentence ever start with “That”? Example: The girl went out in front of a car. That she was running and it was snowing, didn’t help the matter, and she did get struck by the car.
Thank you!
I think you answered your own question, Sue. :)