And it wasn't just any lawmaker.
Mariela Castro, the daughter of President Raul Castro and niece of Fidel Castro, gave the thumbs-down to a workers' rights bill that she felt didn't go far enough to prevent discrimination against people with HIV or with unconventional gender identities.
None of the experts contacted by The Associated Press could recall another "no" vote in the 612-seat National Assembly, which meets briefly twice a year and approves laws by unanimous show of hands.
"This is the first time, without a doubt," said Carlos Alzugaray, a historian and former Cuban diplomat.
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Few in Cuba were even aware of the vote until after the measure was enacted into law this summer, at which point gay activists publicised the vote by Castro, who is the island's most prominent advocate for gay rights.
Arturo Lopez-Levy, a Cuban analyst who lectures at the University of Denver, suggested it might "open doors for other important initiatives."
Mariela Castro herself seemed to hint there could be more debate in the assembly.
"I think we still need to perfect the democratic participation of the representatives within the Assembly." Others are sceptical it will set a precedent.
"I would say that this is more a sign of what Mariela can get away with than a sign of what your everyday parliamentarian can get away with," said Ted Henken, a professor of Latin American studies at Baruch College in New York.
The new labour code bans workplace discrimination based on gender, race and sexual orientation. But it has no mention of HIV status or gender identity.
"I could not vote in favour without the certainty that the labour rights of people with different gender identity would be explicitly recognised," Castro said in the blog interview.
Raul Castro himself has been slowly shaking up Cuba's system by allowing some limited private-sector activity and scrapping a much-loathed exit visa requirement. He's made it clear, though, that the Communist Party will continue to be the only one permitted.