According to the WSJ write-up on the list of voters:
-Perez was supposed to win on the first ballot, but two proxy voters for DNC members that meant to vote for Perez on the first ballot forgot to sign their ballots and were deemed invalid, leading to the first ballot total of 213.5 Perez, 199 Ellison, 12 Boynton Brown, 1 Buttigieg, 0.5 Greene. 214.5 was needed to win, as per the 50% + 1 rule.
-The two proxy voters corrected their mistakes and voted Perez on the second ballot.
-Also the second ballot, ten DNC members (including two from Dems Abroad) switched from Ellison to Perez, four went from not voting to backing Perez, three switched from Perez to Ellison, and 11 switched from Boynton Brown to Ellison.
http://blogs.wsj.com/washwire/2017/03/06/vote-flub-cost-tom-perez-a-first-ballot-victory-in-dnc-chairman-race/
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