The judge dismissed the jury after it rendered its guilty verdict, but forgot that the jury hadn't yet decided whether the defendant had been previously convicted of a serious felony, which was one of the charged enhancements. Before the jury left the courthouse, the judge had the jury call them back in. But the jury didn't actually come back into the courtroom until four hours or so after they'd been discharged, and the judge failed to put on the record what the jury was doing in the meantime. As a result, it was unclear from the record whether they'd discussed the case with outsiders, had talked to the lawyers, etc.
Under such circumstances, yeah, you gotta reverse the subsequent jury's finding that the defendant had a prior felony. You can't reassemble a jury unless you're totally sure they remained under the judge's control the whole time.
That said, all this means is that a new jury's going to be empaneled to decide whether the defendant had previously been convicted. Which I'm certain he has been. So it's going to be the quickest retrial in the history of California jurisprudence.
As well as perhaps the most boring jury service ever.
