The spoiler effect is a term to describe the effect a candidate can have on a close election, in which their candidacy results in the election being won by a candidate dissimilar to them, rather than a candidate similar to them. Different voting systems are affected to a greater or lesser extent by the spoiler effect: in first past the post elections, the spoiler effect can be a defining feature of campaigns, while most Condorcet methods are barely affected by it.
One often cited example of the spoiler effect at work was the 2000 U.S. Presidential election. In that election, George W. Bush and Al Gore had a very close election in many states, with neither candidate winning a majority of the votes. Though Gore won a very modest plurality of the votes nationwide, the mandate of the U.S. Electoral College in combination the Supreme Court decision in Bush v. Gore resulted in the awarding of the presidency to his opponent, George W. Bush. Many Gore supporters contended that the votes that went to Ralph Nader, a popular third-party candidate, would have likely been votes for Gore had Nader not been in the election (though Nader himself argued otherwise). They contend that Nader's candidacy "spoiled" the election for Gore, by taking away enough votes from Gore in many states to allow Bush to win.
