The Catalan Independence Referendum of Oct 1, 2017, certainly has problems enough (deemed illegal beforehand, low participation, police conflicts, etc.), but apart from that - was there voter fraud
Examples:
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La jornada electoral del referéndum de Cataluña ha estado repleta de irregularidades y agujeros negros en el sistema de votación.
The electoral day of the referendum in Catalonia has been full of irregularities and black holes in the voting system.
One guy at four different ballot boxes