In almost all cases, a poisoned jury is a failure of the defense counsel to adequately screen before the trial.
The obvious charge would be that the juror perjured himself when being asked screening questions.
However, just one juror cannot convict. So unless the defense can prove jury packing, which is very hard, the conviction will likely stand.
It is much easier to prove procedural, judicial or prosecutorial misconduct.
No, but for a conviction, they need every single juror, so even one is an issue.