That's not true.... If they say, they can be fair and will base their judgement only on what evidence was presented in the trial, then sometimes the prosecutor, and defense attorney, and judge will accept them....
It happened to me once....I had been exposed to a situation similar to the trial case they were picking jurors for, of which I told the truth about to them, (kinda hoping to be recused cuz I had an awful lot of work to do at my job), but they asked me if I could be fair and only use evidence presented in the trial, and I said yes, I'm certain I could do that....
And I was sent to another room to wait to be dismissed, but about an hour and a half later I was called back in and told I was in the jury!!