A big thanks to David for passing this along. It is applicable to investing in many ways. This is from The Intelligent Investor (Revised Edition).
“In the Air Force we have a rule: check six. A guy is flying along, looking in all directions, and feeling very safe. Another guy flies up behind him (at "6 o'clock"--"12 o'clock" is directly in front) and shoots. Most airplanes are shot down that way. Thinking that you're safe is very dangerous! Somewhere, there's a weakness you've got to find. You must always check six o'clock.” --U.S. Air Force Gen. Donald Kutyna