How many of those were actually effective? Difference is regular hardball politics and game changing stuff. A surprise in an off election year will be spread over many candidates - not just one.
The 1992 Bush v Clinton race was probably swung by the indictment of Caspar Weinberger. Clinton was the "comeback kid" who consistently ran in last place during the Democrat primaries, but he surged ahead of Bush after the indictment.
We know that the 2000 Bush v Gore race was heavily influenced by the DUI announcement. Bush would have likely won outright had it not happened. The October Surprise caused the Florida situation to happen, which lead to the eventual systemic program of the Left to undermine the Bush presidency as "selected, not elected."
The 2006 midterm Foley incident lead to the Nancy Pelosi era in the House. Democrats might have picked up seats, but Pelosi turned the Foley incident into a "culture of corruption" that lead to the ouster of Denny Hastert and others.
In 2010, Meg Whitman had a good shot at winning the Governor's race in California until Gloria Allred marched out the "victim" maid who claimed that Whitman was a cheap multi-millionaire boss who hired illegal aliens.
All of Barbara Boxer's races swung on her late opposition research info drops.
-PJ