FDR's presidency was marked by more than a little chutzpah—22nd Amendment, anyone?—but there was perhaps no move more purely, willfully brass-balled than his personal tax scheme, described by Sarah Lawsky at TaxProfBlog:
Throughout his first term, President Franklin Roosevelt paid taxes at the rates in effect when he took office, even as statutory tax rates increased. His position was that paying tax at a rate higher than that in effect at his inauguration reduced his salary, which violated the Constitutional provision that states that the president's compensation "shall be neither increased nor diminished during the period for which he shall have been elected."
From Roosevelt's 1937 return [PDF], this note:
"I am wholly unable to figure out the amount of tax for the following reason," he writes. "The first twenty days of January, 1937, were part of my first term of office and to these twenty days the income tax rates as of March 4, 1933 apply. To the other 345 days of the year 1937, the income tax rates as they existed on January 30, 1937. As this is a problem in higher mathematics, may I ask that the Bureau let me know the amount of the balance due?"
Among execs, Roosevelt seems to have been alone in trying out this line of reasoning. Judges, who enjoy the protection of similar constitutional language about their compensation, have tried the Roosevelt gambit too. In the 1920s, the Supreme Court ruled that judges' salaries, including their own, should be tax exempt. But by 1939 and again in 2001, the Court decided their paychecks were taxable after all, suggesting that they wouldn't go for this reasoning if Obama tried it on them these days.
See a page of FDR's return here.