This from a recent Reuters article “New York City watchdog decries 'irregularities' in primary voting”:
A New York City official on Tuesday ordered an audit of the city's election authorities, citing "deep concern over widespread reports of poll site problems and irregularities" as voters cast their ballots in the state's primary election.
...
Of particular concern, Stringer said, were allegations of widespread removal of eligible voters from registration rolls and incorrect party affiliations on voter records. Stringer said eligible Democratic voters in Brooklyn fell by 120,000 from November 2015 to April 2016 without explanation.
And, of course, Brooklyn is Sanders’ hometown, where he drew the biggest crowd in his campaign.