Update: Monday, Aug. 6
Rachel Feintzeig and Vanessa Fuhrmanns write in the Wall Street Journal, "Past social media posts upend hiring: Employers grapple with screening job applicants' online personas, including years' old tweets". I can say, "I told you so" on this one. The article talks about the cases above, as well as widespread problems.
Update: Wednesday, Aug. 8
Read Ezra Klein's, "The Problem with Twitter Wars, Explained: Behind our Twitter wars lies Twitter's problems." It's possible to set up your account to delete old tweets. I doubt many employers actually mine old tweets to look for problems, but it's possible to imagine setting up an HR contracting company to do just that!! You can also have old Facebook posts deleted automatically. Not sure about Instagram. This is one reason why some people prefer Snapchat. I don't feel that way for myself, but I see how many people do.
James Gunn now seems to be in demand again (MSN).
Update: Friday, Aug. 17
Victor David Hanson has a detailed op-ed in the Washington Times Aug. 16 that covers the problem pretty well, here.