Sen. Claire McCaskill has now chalked two major flubs up to sloppiness.

The Missouri Democrat was forced to come clean on Tuesday after falsely claiming she was being treated for breast cancer when the Senate voted on the Ensuring Patient Access and Effective Drug Enforcement Act in 2016 and "did not go along" with it. That legislation is facing fresh scrutiny this week after a "60 Minutes" report revealed it weakened the Drug Enforcement Agency's ability to go after drug distributors.

A McCaskill spokesperson said the untruth "was sloppy on our part and we take responsibility." The senator herself later called her false claim a "sloppy embarrassing mistake."

If that sounds familiar, it's because McCaskill used the same language to describe her ethical missteps in the heat of the 2012 campaign season.

Dogged by questions about her private plane, McCaskill revealed she and her husband failed to pay $320,000 in back taxes on the $2 million aircraft (which she also bizarrely quipped "normal people can afford"). That was just a "big, serious, sloppy mistake," the senator said at the time in 2011.

Facing reelection in a state President Trump won by nearly 20 points, McCaskill can't afford to make many mistakes. Multiple analysts have suggested she's the most vulnerable incumbent going into the 2018 midterms. Conceding to sloppiness could come across as refreshingly honest once or twice, but voters may start to wonder whether it's actually masking incompetence, or worse, dishonesty.

Emily Jashinsky is a commentary writer for the Washington Examiner.