Bernie Sanders says Hillary Clinton isn't qualified to be president. He says he only said this because Hillary Clinton said he's not qualified to be president, which Clinton's campaign says she didn't say. Asked for evidence of Clinton's attack, Sanders's campaign pointed to this Washington Post story, which is headlined, "Clinton questions whether Sanders is qualified to be president," but does not actually include any comments from Clinton questioning Sanders's qualifications to be president.
Thus goes presidential politics in the most powerful country the world has ever known in the year of our lord 2016.
A few points:
1) The debate about who is "qualified" to be president is frustrating because Americans have never decided what it means to be qualified for the presidency. In practice, we tend to elect career politicians to the office despite occasionally flirting with businessmen, but there's no rule saying we have to do that.
2) That said, there's no doubt that both Hillary Clinton and Bernie Sanders are more than qualified to be president. Both of them have been in politics longer than the current president of the United States, for instance.
3) Both Clinton and Sanders have significantly more government experience than the current frontrunner for the Republican nomination, who has spent exactly zero days in elected office or any other kind of public service.
4) Even though voters prefer politicians, they don't much respect political experience. Note that Joe Biden is the vice president, not the president, and John McCain wasn't elected to the White House. Which is all to say that passing the amorphous, ill-defined "qualifications" bar doesn't win you the presidency. This is a debate that's rather less important than it sounds.
5) Until now, the Democratic race has been pretty clean. But the tone is getting harsher as the campaign drags on. This particular attack from Sanders might prove consequential, because if Clinton wins the nomination, calling her unqualified for the presidency is going to make it rather awkward for Sanders to turn around and endorse her for that same office.
6) Awkward isn't the same as impossible, of course. Remember that Lindsey Graham endorsed Ted Cruz after joking that most of his fellow senators hate Cruz so much they would look the other way if he were murdered on the Senate floor.
But Sanders is a candidate who takes his personal integrity very seriously and whose ties to the Democratic Party are already fairly weak. The more things he says about Clinton that are hard for him to walk back, the easier it becomes for him to decide to simply not walk them back after a bruising, frustrating campaign that leaves him more disillusioned with the Democratic Party establishment than ever.