It sounds almost Rumsfeldian: Know what you know and know what you don't know.

Practical application: I wouldn't go on a TV show or do an interview to detail the intricacies of the Linux operating system. The extent of my knowlege of Linux is a few command line queries left over from the AIX OS that ran on the mainframe back at Cal Poly -- when I was an electrical engineering major. [I do know that there are Linux OS snobs and that they can quickly become boring. ;-P ]

Parallel practical application for John Cougar Mellencamp: Don't go on the Charlie Rose Show and talk about politics.

You can watch the video at the link, but I'll summarize: We shouldn't have gone after Osama bin Laden after 9/11, we should've talked with him. Though Mellencamp doesn't know a lot about the Japanese attack on Pearl Harbor, we probably shouldn't have gone after them in the wake of that attack either.

Better to be thought a fool than to open one's mouth and remove all doubt.