Answer:
One notable example is the 2000 presidential election between George W. Bush and Al Gore, where the election results in Florida were disputed and ultimately decided by the Supreme Court. Additionally, in the 2016 presidential election, Donald Trump repeatedly claimed that the election was rigged and refused to commit to accepting the results if he lost. However, after winning the election, Trump did not challenge the results and took office peacefully.
It is also worth noting that while there may be claims of evidence showing that an election was not free and fair, it is important to rely on verified and credible sources to determine the validity of such claims. The vast majority of elections in the United States have been conducted fairly and without evidence of significant fraud or irregularities.