- In Georgia, after the 1992 Senate election where incumbent Democrat Wyche Fowler lost a runoff with Republican Paul Coverdell, the Dems in the legislature changed the law only requiring a runoff if the winner had less than 45%. Max Cleland won his race in 1996 with only 48.9% of the vote. Then after Republicans got the legislature in 2005, they changed it back to 50%. link
- In Massachusetts, the Dem legislature didn't want Mittens picking John Kerry's replacement if he won the Presidency so they changed the law to just make the Senate seat vacant until a special election. Obviously Kerry didn't win and then their law almost screwed them in 2009 when Ted Kennedy was dying. They rushed to change the law back to a gubernatorial appointment just before he died. Now that Charlie Baker is governor and if Elizabeth Warren is chosen to be in a Biden cabinet, the law might be changing again. link
- Minnesota's laws regarding a death or withdrawal of a candidate before the general election were changed after the death of Sen. Paul Wellstone in 2002 (and probably also indirectly because of Jon Grunseth in 1990). If a "major party candidate" dies/withdraws, the regular general election is voided and a special election would be held afterwards. This of course brings us to today in MN-2 where a single-issue pro-marijuana candidate (and potential GOP plant) died suddenly, the seat may be vacant at the start of the new Congress, and incumbent Democrat Angie Craig may have to run in a low-turnout special election though she may have already won the voided general election through no fault of her own. link
If you have another example, please post it.