The incorrect use of and punctuation surrounding the words "that" and ", which" drives me to distraction. I am currently reading a book, which was recommended to me, that is loaded with the incorrect use of these and it is irritating!
This website is useful and gives examples:
http://iws.ccd.edu/jmiller/A%20Brief_%20No-Nonsense%20Guide%20to%20Comma%20Usage.htm