These are the funniest things I've heard professors joke about. You can always add on to this list.
http://www.ranker.com/list/funniest-things-college-professors-like-to-make-fun-of/stokified, people, college, Humor, education, other,
People Texting During Class
Internet Explorer
Tell any computer science professor you like IE. Wait for the facial expression.
People Late to Class
Yahoo
People Who Don't Go to Class