I don't know if this one has been mentioned yet, but misuse of which and that. Which signals that what follows is not essential to the meaning of the sentence. And a comma must always appear before the word which.
That signals that what follows is essential to the meaning of the sentence.
For example: The car that ran the red light was purple. The focus here is identifying the car that went through the red light, i.e, the purple car. The car, which ran the red light, was purple. The focus here is naming the color of the car. Running the red light is incidental.
I'm getting off my soapbox now.