From Eisenhower on, every two-term president has had a White House scandal in the 2nd term. Not a policy failure, but a weird personnel/legal issue boil up out of the White House itself.
Eisenhower's chief of staff, Sherman Adams resigned in 1958 after it was discovered that he had taken gifts from and made calls to the Federal Trade Commission and the Securities and Exchange Commission about Boston textile manufacturer Bernard Goldfine.
Nixon of course had Watergate. Reagan had Iran-Contra. Clinton had the Lewinsky imbroglio and George W. Bush had the Scooter Libby crisis. All of these scandals involved White House personnel acting badly in ways that touched on the president himself while in office.
White House scandals, like, Tolstoy's unhappy families, are unique. The Eisenhower scandal was about money, while Nixon was about power. Clinton's scandal was more "ahem" personal - the oldest foible in the book. Reagan's scandal was about conducting an alternate foreign policy. The Bush 43 contretemps (it seems to me at least) had a whiff of revenge - the sense that someone should pay somehow for getting the U.S. into the Iraq War. It is tough to see a common pattern. Sometimes staffers went a bit too far interpreting the president's instructions (that's a charitable understanding of Watergate and Iran-Contra.) In other cases, specific individuals just, well, got up to stuff they shouldn't have.
So no pattern suggests itself with these scandals. No generalizations present themselves, just that after about six years something is going to come up/go wrong.
So far, Obama has not had such a scandal and the clock is ticking. Of course breaking this pattern, just as Reagan broke the 20 year curse (every president elected in an 0 year had died in office starting with William Henry Harrison in 1841 and extending to JFK in 1963) would be a great thing.
I know partisans on the other side will insist that Obama has escaped scandal due to media bias or no one looking hard enough. But, while the Obama White House is undoubtedly up to stuff (politics is a rough business after all) when a real White House scandal comes stuff really will stick.
Of course, maybe something will boil up. But Obama has had pretty strong chiefs of staff in Rahm Emmanuel and Denis McDonough. While Obama has been criticized for surrounding himself with loyalists and cosseting himself in a tight inner circle - one advantage of this is discipline. Unfortunately there is little suggest that this discipline has resulted in good policy.
Another lesson worth noting.