In 1968 before the Democratic Convention riots.
The eventual nominee, Hubert Humphrey, hadn't won any primaries. One of the most popular candidates had been Robert Kennedy, but he was assassinated by Sirhan Sirhan in Los Angeles right after Kennedy had won the California primary.
I don't wish for that kind of thing to happen to any candidate, regardless of how I feel about their politics, but I can see a front runner dropping out for other reasons - an FBI investigation resulting in an indictment, for example.
Even without that kind of drama, the Democratic Party is looking at a widening division. We've already seen how Sanders supporters have shut down Trump's rally in Chicago earlier this year.
And shut down the road to the rally location in Arizona.
And blocked Trump's motorcade in California.
Which Trump got around by getting out and walking, like a boss.
What do you think they're going to do if Hillary Clinton gets the nomination? Even in the somewhat-less-likely event that Sanders gets the nomination, do you think all of these people will just say, "our job is done" and go home to wait for the election? Nope. It's going to be more of this until November and it's going to turn "Never Trump" people who are watching the chaos into Trump voters.