The error means that Charles Mapps won't be going to prison for shooting his girlfriend, at least until after he stands trial a second time.
Although the 13th juror dutifully sat through the testimony and the 45-minute deliberation, District Judge Mark Ellis said the extra hand could be considered an undue influence.
"In 23 years I've never seen anything like this," Ellis said. "The jurors all seemed pretty upset, but there's no way to un-ring that bell."
The Houston Chronicle said Wednesday that the heat was falling squarely on a substitute bailiff who apparently failed to do anything when the extra juror tried to leave and never informed Ellis about the problem.
"I told him I never want to see him in my courtroom again," Ellis said.