I don't accept the results of the 1968 election. Nixon won by committing treason and sabotaging LBJ's peace negotiations in Vietnam.
I don't accept the results of many Republican wins, which were caused by everything from Gerrymandering to voter suppression. I'm also against the immoral role that 'big money' plays, though that doesn't change the fact of the votes.
I don't accept the results of the 2000 election, which Gore not only won in an accurate count, but in which Republicans removed up to 100,000 mostly Republican voters in Florida compare to the official 534 vote margin, not to mention thousands lost from a ballot error in a county. While the 2004 election wasn't wrongly decided, there were problems such as Ohio, and it's invalidated by the 2000 election.
I don't accept the results of the 2016 election, in which there's a good case that Russian interference was enough to change the result; and take issue with other things in the election and the primary, including various inappropriate measures to help Hillary in the primary and huge media coverage bias.
I take a lot of issues with the recall of CA Governor Gray Davis at the behest of Enron to get them a sweetheart deal for defrauding billions from CA by replacing him with Arnold Schwarzeneggar, but don't dispute the election result.
I'm not pleased how the 1876 election went, but. I think that's the list of invalid election results, barring ongoing issues that warp the results.