On May 28, there was a "flash crash." If you didn't notice it, that is because it occurred not in 2010, but in 1962. Its aftermath offers some clues on what might happen in the wake of this year's flash crash on May 6.

"The stock market careened downward yesterday," reported The Wall Street Journal on May 29, 1962, "leaving traders shaken and exhausted." The Dow Jones Industrial Average fell 5.7% that day, down 34.95, the second-largest point decline then on record.

"The drop took place on volume so heavy," added the Journal, that the "ticker wasn't able to finish reporting floor transactions until 5:59 p.m., two hours and 29 minutes after the market closed."

Like this year's flash crash, the "market break" of 1962 came after a run-up in the market that had led many investors into complacency. In 1961, stocks had risen 27%, with leading technology stocks like Texas Instruments and Polaroid trading at up to 115 times earnings.

Then, without warning, stocks "broke."