Hillary Clinton: Giftgate (2001)
Former Clinton adviser Dick Morris not only has said he'd leave the country, if she was elected President (that's how much the people that know her, respect her), but he alleged in a New York Post article that the former first lady failed to report many dresses, and purses and jewelry that she (or Chelsea) was gifted while First Lady. Clinton aides defended her by claiming some of the gifts were before innaugeration (so don't technically count, even if it's still ethically questionable). After shining the light on the issues, others were either declared or given to the national archives, there's no evidence she would have done that otherwise.
Nancy Reagan had supposedly got caught in a similar thing (on a smaller scale) back in the 1980's, which makes this worse. Because it was a KNOWN issue now. Ironically one of the defenses of Lootergate (2001) by her water carriers at PolitiFact, was that Nancy had done it with couture. But if they had any sense of journalistic curiosity, they would have found that Hillary was doing that different crime (Giftgate) as well. Thus their excuse that since Nancy committed a different mistake, that this forgave Hillary, ignores that Hillary did that "mistake" too, after having more opportunities to know better.
Issues: 2nd Amendment - Hillarycare - It takes a Village - SecState