Every U.S. presidential candidate fears an "October surprise." This is an important development, inimical to the candidate, that materializes just before voters go to the polls. Bad news at this late date is thought to negatively influence voters at a point that is too late to correct. Last Friday, Democratic nominee Hillary Clinton got her October surprise when FBI Director James Comey announced that his organization had found email that might be related to the investigation of Clinton's email practices, an investigation that concluded last summer that while the candidate had been sloppy, she had not acted in ways that justified prosecution. The FBI is examining the discovery to see if it changes that conclusion and puts Clinton in jeopardy.
The original October surprise occurred in 1972, when 12 days before the election, U.S. National Security Advisor Henry Kissinger announced that "peace was at hand" in negotiations with North Vietnam to end that bloody war. Democratic challenger George McGovern never had much of a chance in that election, but his candidacy stemmed from opposition to the war; Kissinger's announcement denied him a platform to oppose President Richard Nixon and McGovern was crushed in the vote that followed.
The incident that looms largest in the U.S. imagination occurred in the 1980 campaign, when Republicans worried that then President Jimmy Carter would engineer the release of American hostages held in Iran just before the vote to influence the result. That never materialized; in fact, the hostages were released only after Ronald Reagan took the oath of office to ensure that Carter was utterly humiliated for his support for the shah of Iran.